(Reuters) - The United States and Europe were planning new sanctions on Tuesday to punish Moscow over civilian killings in Ukraine, and President Volodymyr Zelenskiy warned more deaths were likely to be uncovered in areas seized from Russian invaders.

Russian forces withdrew from towns north of the capital Kyiv last week as it turns its assault to Ukraine's south and east. Ukrainian troops recaptured towns devastated by nearly six weeks of war, including Bucha, where dead civilians lined the streets.

Searing images of a mass grave in Bucha and the bound bodies of people shot at close range drew an international outcry on Monday.

U.S. President Joe Biden called for a war crimes trial against Russia's President Vladimir Putin and the United States will ask the U.N. General Assembly to suspend Russia from the Human Rights Council. 

Russia denied any accusations related to the murder of civilians and said it would present "empirical evidence" to a meeting of the United Nations Security Council on Tuesday proving its forces were not involved.

In an early morning video address, Zelenskiy said he would also address the Security Council on Tuesday as he builds support for an investigation into the killings in Bucha.

"And this is only one town. One of many Ukrainian communities which the Russian forces managed to capture," Zelenskiy said. "Now, there is information that in Borodyanka and some other liberated Ukrainian towns, the number of casualties of the occupiers may be even much higher," he added, referring to a town 25 km (16 miles) west of Bucha. 

Reuters saw several bodies apparently shot at close range, along with makeshift burials and a mass grave in Bucha, but could not independently verify the number of dead or who was responsible.

Ukraine's foreign minister, Dmytro Kuleba, said he spoke with U.N. Secretary-General Antonio Guterres about Bucha and stressed "that Ukraine will use all available UN mechanisms to collect evidence and hold Russian war criminals to account." 

Kuleba also spoke with his Chinese counterpart Wang Yi in a phone call on Monday, with Beijing again calling for talks to end the conflict in Ukraine.

The call, which Beijing said was made at Ukraine's request, was the first reported high-level conversation between the countries since March 1, when Kuleba asked Beijing to use its ties with Moscow to stop Russia's invasion, the Ukrainian foreign ministry said at the time.