Boris Johnson has condemned Russia’s “despicable attacks” against Ukrainian civilians in Irpin and Bucha, adding that “we will not rest until justice is served”.
The Prime Minister said “no denial or disinformation from the Kremlin” can conceal that President Vladimir Putin is “desperate” and “his invasion is failing”.
An adviser to Ukrainian president Volodymyr Zelenskyy earlier said dead civilians had been found on the streets of the small city of Bucha and the Kyiv suburb of Irpin, in what resembled a “horror movie”.
The Associated Press said its journalists in Bucha north west of Kyiv watched as Ukrainian soldiers used cables to drag bodies off a street – from a distance due to fears they may be booby-trapped.
In a statement, Mr Johnson said the UK is “stepping up” its sanctions and military support, and “bolstering” humanitarian help for those on the ground.
A woman walks on a street, amid Russia's invasion on Ukraine, in Bucha, Ukraine Mikhail Palinchak
Footage released by the Defence of Ukraine in Bucha, near Kyiv, which appears to show dead bodies lying in the street after Russian troops left the region Defence of Ukraine
He added: “The UK has been at the forefront of supporting the International Criminal Court’s investigation into atrocities committed in Ukraine.
“The Justice Secretary has authorised additional financial support and the deployment of specialist investigators – we will not rest until justice is served.”
Liz Truss said the UK will “not rest” until those responsible for atrocities in Ukraine have faced justice, adding that Russia will not be allowed to cover up its involvement through “cynical disinformation”.
Ms Truss referred to the “appalling acts” carried out by Russian forces in the towns of Irpin and Bucha.
The Associated Press said its journalists in Bucha, north-west of Kyiv, watched as Ukrainian soldiers used cables to drag bodies off a street, done so from a distance due to fears they may be booby-trapped.
AP said its journalists counted at least six bodies, adding that locals said they were civilians killed without provocation by Russian soldiers.
Russia has denied that Russian forces were responsible for the killing of civilians in the Ukrainian town of Bucha.
Below is the full statement by the Russian defence ministry. It is an unofficial translation from Russian.
"All the photos and videos published by the Kyiv regime, allegedly testifying to the 'crimes' of Russian servicemen in the city of Bucha, Kyiv region, are another provocation.
"During the time that Russian armed forces were in control of this settlement, not a single local resident suffered from any violent actions. Russian servicemen delivered and issued 452 tons of humanitarian aid to civilians in the settlements of the Kyiv region.
A shot car with inscriptions "Children" is seen on the street, amid Russian invasion on Ukraine, in Bucha, Kyiv region Gleb Garanich
"For the entire time that the city was under the control of the Russian armed forces and, and afterwards, until today, in Bucha, local residents freely moved around the city and used cellular communications.
"Exits from the city of Bucha were not blocked. All local residents had the opportunity to freely leave the settlement in the northern direction, including to Belarus.
"The southern outskirts of the city, including residential areas, were shelled around the clock by Ukrainian troops from large-caliber artillery, tanks and multiple rocket launchers.
"We would like to emphasise especially that all Russian units completely withdrew from Bucha on March 30, the day after the face-to-face round of negotiations between Russia and Ukraine in Turkey.
"Moreover, on March 31, the mayor of Bucha, Anatoliy Fedoruk, confirmed in his video message that there was no Russian military in the city, but he did not even mention any local residents shot in the streets with their hands tied.
"Therefore, it is not surprising that all the so-called 'evidence of crimes' in Bucha appeared only on the fourth day, when SBU [Security Service of Ukraine] officers and representatives of Ukrainian television arrived in the city.
"Of particular concern is the fact that all the bodies of people whose images were published by the Kyiv regime, after at least four days, have not stiffened, do not have characteristic cadaveric spots, and blood has not turned up in the wounds.
"All this irrefutably confirms that the photos and video frames from Bucha are another staged performance by the Kyiv regime for the Western media, as it was in Mariupol with the maternity hospital, as well as in other cities."