Girls aged as young as 14 have reported being raped by Russian soldiers, a Ukrainian official has said.
Lyudmyla Denisova, the Ukrainian parliament's human rights commissioner, said 25 women and girls, aged from 14 to 24, had called a helpline reporting they had been raped in Bucha.
The town just outside of Kyiv has become the scene of alleged war crimes, leading Boris Johnson to say the actions of Russian forces in Ukraine appeared close to “genocide”.
Denisova added Russian forces were using rape as "the new weapon", telling the BBC: "That was happening for a month.
"We will keep documenting these terrible crimes, unfortunately, and every criminal will be punished."
Watch: Actions of Russian forces in Bucha 'doesn't look far short of genocide', says Boris Johnson
UN political affairs chief Rosemary DiCarlo said UN human rights monitors were seeking to verify allegations of sexual violence by Russian forces.
"These include gang rape and rapes in front of children," she said.
"There are also claims of sexual violence by Ukrainian forces and civil defense militias."
Russia has refuted claims of mass killings since Bucha was freed from their control on 31 March.
Russian foreign minister Sergei Lavrov claimed footage and images of bodies in the streets were "staged" and Ukrainians had used "fake dead bodies".
However, satellite imagery of the area in the days before Bucha was freed shows at least nine bodies lying in the street for weeks, contradicting Russian claims.
Tied bodies shot at close range were found after Russian troops withdrew, and other bodies were found in a mass grave at a church.
Kyiv is looking into reports of alleged atrocities in Bucha, with Ukrainian president Volodymyr Zelenskyy telling the United Nations Security Council on Tuesday "accountability must be inevitable" for Russia as he accused it of committing "the most terrible war crimes" since the Second World War.
Zelenskyy showed a short video of burned, bloodied and mutilated bodies, including children, in Irpin, Dymerka, Mariupol and Bucha, where Ukraine accuses Russian troops of killing hundreds of civilians.
Denisova added more than 400 residents were missing in the town of Hostomel, near the capital Kyiv, after a 35-day occupation by Russian forces, and quoted witnesses as saying some of them had been killed.
Britain and other Western allies have unveiled sweeping new sanctions against Russia.
The UK is freezing the assets of Sberbank, Russia’s largest bank, and Credit Bank of Moscow, banning all new investment in Russia and targeting eight more oligarchs.
Foreign secretary Liz Truss also announced Britain would end all imports of Russian coal as well as oil by the end of the year.
The announcement coincided with a parallel move against Sberbank by the US which is also sanctioning Vladimir Putin’s two adult daughters, Mariya Putina and Katerina Tikhonova, and other members of the Russian elite.
Earlier the European Commission set out proposals for a fifth round of sanctions, including a ban on coal imports, for approval by EU ambassadors.
Russia refers to the invasion as a "special military operation" designed to "denazify" Ukraine.