Responding to an interview with one of the victims, the prime minister said on social media: "This is a harrowing and brave testimony.
"The girls at the heart of this case have shown extraordinary bravery and strength in heinous circumstances.
"This is an appalling case and it is right that law officers are urgently reviewing the sentences."
Two girls were raped in separate attacks in Fordingbridge, Hampshire, by two 15-year-old boys who were given non-custodial sentences.
The judge said he wanted to "avoid criminalising these children unnecessarily" and gave them youth rehabilitation orders (YRO) and intensive supervision and surveillance (ISS).
The attorney general is reviewing the case and could refer it to the Court of Appeal, who might then choose to change the sentences, if they feel they are unduly lenient.
The boys recorded the rapes - in November 2024 and January 2025 - and shared some of the footage, Southampton Crown Court heard.
One of the victims told the BBC's Laura Kuenssberg programme she was devastated by the sentence.
"The words hit like a rock straight in my face," she said.
"He [the judge] almost made it seem as if what they boys did was not OK, but it was OK in the eyes of the law because they were still children."
The sentences have been criticised across the political spectrum.
Conservative Party leader Kemi Badenoch called it a "disgrace", while Reform's Robert Jenrick said the judge had made a "very bad error".
The prime minister's comments come after the girl's mother appealed to him directly in the BBC interview.
"If it was your daughter, your niece, your son, your nephew, your family member, would you be happy?" she said.
"Because we're not happy and I don't think any other member of the public will be happy too. So you're in a position of power to help, so please help."
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Judge Nicholas Rowland praised the girls' "bravery" for giving evidence, but told the two young rapists: "I have to remember that you are not small adults.
"I have to think how likely you are to do serious things again and I need to make sure you do not do serious things again in the future."
He also said "peer pressure" had played a large part in their actions.
The court heard one had ADHD and "long-standing anxiety", while the other also had an ADHD diagnosis and an IQ in the "bottom 1%".
A third boy,14, described as having "mild cognitive impairment", was also given a YRO over encouraging one of the others during the second rape.
Two people were killed in the capital overnight, according to mayor Vitali Klitschko, while Kyiv's regional governor added two others were killed in surrounding districts.
Ukrainian president Volodymr Zelenskyy said Russia said on Sunday a Oreshnik intermediate-range ballistic missile (IRBM), capable of carrying nuclear or conventional warheads, was used in the attack.
The Russian Defence Ministry later acknowledged it used the hypersonic missile in the strikes, along with its Iskander, Kinzhal and Zircon missiles.
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Mr Zelenskyy added there were at least 83 people injured in the strikes, which involved 600 drones and 90 missiles, mainly targeting Kyiv.
He also said that Russia had struck the city of Bila Tserkva in the Kyiv region with the Oreshnik.
The president said on Saturday that US and Western intelligence were suggesting Russia was planning to use the hypersonic missile in the strikes overnight.
Russia said the strikes were in response to Kyiv's attacks on civilian targets in Russia. On Saturday, Russian officials said 18 people were killed in a Ukrainian drone strike on a student dorm in the Russian-controlled Luhansk region.
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Ukraine's military denied responsibility for the attack, saying it had struck an elite drone command unit in the area.
Kyiv military administration head Tymur Tkachenko said on Telegram that damage was recorded across 40 locations across several districts of the city, including residential buildings.
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Russia last launched an Oreshnik against Lviv on 8 January as part of an intensive overnight attack on western, central and southeastern Ukraine, comprising 278 Russian missiles and drones.
Mr Klitschko added that a school building being used as shelter by residents was damaged, while local authorities reported supermarkets and warehouses across the city were also damaged.
The shuttle service was travelling through the city of Quetta when a vehicle packed with explosives hit one of the carriages in a residential area, with people living nearby among the dead.
Two of the train carriages overturned and caught fire due to the force of the blast, sending black smoke into the air, according to footage online.
More than 70 others were injured in the explosion in the southwestern province of Balochistan, according to government and security officials.
Doctors at local hospitals said 20 people were in a critical condition.
The incident took place in an area where security forces are usually stationed.
Several buildings were damaged along with more than a dozen vehicles parked along the road, according to witnesses and images circulating on social media.
The train was carrying passengers from Quetta's cantonment area to connect with the Jaffar Express long-distance train when the blast happened.
Pakistan's Prime Minister Shehbaz Sharif strongly condemned the incident, calling it a "cowardly act of terrorism" in a post on X and offering condolences to the families of the victims.
Shahid Rind, a spokesperson for the Balochistan provincial government, said they "strongly condemn the targeting of innocent civilians and are deeply saddened by the loss of precious human lives. Terrorist elements deserve no leniency".
A medical emergency was declared at hospitals in the city following the incident, and an investigation has been opened, he said.
Quetta is the capital of insurgency-hit Balochistan province, which borders Iran and Afghanistan.
The outlawed Baloch Liberation Army, (BLA), which demands independence from Pakistan's central government, has claimed responsibility for Sunday's attack in a statement sent to reporters.
The militant group said it targeted a train carrying security personnel, the Associated Press said.
It is one of the separatist groups in the oil and mineral-rich region, where insurgents have often targeted security forces, government installations and civilians as well as elsewhere in the country.
The BLA has previously claimed it was behind similar attacks.
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Authorities in Pakistan have claimed the insurgency has been suppressed, but violence in Baluchistan has persisted.
At least 26 people, including soldiers, were killed in November 2024 when a suicide bomber attacked a train station there.
Rescuers have been searching through the rubble of the nine-floor construction for another 21 people who are missing after Sunday's disaster.
A 65-year-old Malaysian national, whose body had been recovered from a neighbouring hotel affected by the collapse, was the first person confirmed dead.
Maria Leah Sajili, from the regional Bureau of Fire Protection, told reporters that, of the 21 people reported missing, five people were confirmed trapped.
Two of the five were in contact with rescuers, but the status of the others was not immediately clear.
Rescue efforts, using trained rescue dogs, heartbeat detection devices and heavy equipment to sift through the debris, have continued, but operations will become more challenging after dark, Ms Sajili said.
Officials said an investigation had been launched into what caused the collapse in the city of Angeles, north of the capital, Manila on Luzon, the nation's largest and most populous island.
Rescuers have had to climb over a mound of concrete slabs and mangled steel, covered in green netting, in their search for survivors.
A total of 24 people have been pulled out of the site, a figure that includes others who were nearby.
Those rescued included a 51-year-old Malaysian national who was staying in a nearby budget hotel.
It was damaged when the concrete structure came down, Jay Pelayo, the Angeles city information officer, was quoted as saying by Reuters.
Mr Pelayo had earlier told DZBB radio that he believed 30 to 40 people were feared trapped, after he talked to a site foreman who escaped.
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According to records, a 10th floor was being built to put a pool on top of the approved condo-hotel.
Ambulances were on standby, and fire engines had been sent to help in the rescue effort, Mr Pelayo said, adding that moving the concrete debris was hampering rescuers.
Ayrton Redfearn, 23, from Devon, died on 9 May while fighting in the eastern Donetsk region.
He had joined a specialist unit with Ukraine's military last year and was due to receive a bravery award for saving someone's life five days before his death, his mother, who gave her name as Natasha, said.
Sharing a tribute on Facebook, his mother said Mr Redfearn was "extremely happy" in Ukraine and made friends with people from around the world "all with the same mission of fighting for Ukraine".
He had been known by the call sign "Musso" and was made a team leader earlier this year.
His mother added: "Huge tributes have come in from Ukraine from Ayrton's commander and his brothers (comrades) about his exceptional service and being an outstanding soldier, whilst remaining the clever, funny, prankster whilst off duty, always able to boost morale, and 'he was 100% locked in and competent when it was time to go to work'."
A Foreign, Commonwealth and Development Office (FCDO) spokesperson said: "We are supporting the family of a British national who has died in Ukraine, and are in contact with the Ukrainian authorities."
The FCDO warns that British nationals fighting in Ukraine face a high risk of maltreatment.
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Its website says: "British nationals fighting in Ukraine have been killed or captured. British nationals undertaking humanitarian work have also been detained by Russian authorities.
"The risk to life, or of maltreatment, is high."
Early on in the war, Ukraine's authorities said more than 20,000 people from 52 countries came to Ukraine's aid.
Since then, the number of foreign fighters in Ukraine's military has been classified.