British Prime Minister Boris Johnson said "despicable attacks against civilians" in Bucha and Irpin near Kyiv were evidence that Russia was committing war crimes in Ukraine, and that Britain would step up sanctions and military aid in response.
"I will do everything in my power to starve (Russian President Vladimir) Putin's war machine," Johnson said in a statement on Sunday.
"We are stepping up our sanctions and military support, as well as bolstering our humanitarian support package to help those in need on the ground," he said.
Germany's defense minister said earlier on Sunday that the European Union must discuss banning the import of Russian gas after Ukrainian and European officials accused Russian forces of committing atrocities near Kyiv.
"There has to be a response. Such crimes must not remain unanswered," the defense ministry quoted Christine Lambrecht as saying in an interview with the public broadcaster ARD.
Berlin has so far resisted growing calls to impose an embargo on energy imports from Russia, saying its economy and that of other European countries are too dependent on them. Russia supplies 40% of Europe's gas needs.
But Lambrecht said EU ministers would now have to discuss a ban, according to a tweet from her ministry.
Foreign Minister Annalena Baerbock on Sunday also called for tougher sanctions on Moscow but stopped short of mentioning the energy sector.
"Those responsible for these war crimes must be made accountable. We will tighten the sanctions against Russia and will assist Ukraine even more in defending itself," she said on Twitter.
The EU has been working on additional sanctions for some time but Economic Commissioner Paolo Gentiloni said on Saturday that any additional measures would not affect the energy sector.
Ukraine said on Saturday it had taken complete control of the Kyiv region for the first time since Russia launched its invasion on Feb. 24. The mayor of Bucha, a liberated town 37 km northwest of the capital, said 300 residents had been killed by the Russian army.
Russia's defense ministry denied the allegation, saying footage and photographs showing dead bodies in Bucha were "yet another provocation" by Kyiv.
German Chancellor Olaf Scholz called in a statement for international organizations such as the International Committee of the Red Cross (ICRC) to be allowed access to the affected areas to independently document what he described as atrocities.