After television footage showed mutilated bodies of people in civilian clothes some with their hands bound behind their backs in Bucha, Ukraine, the World Council of Churches head said the reports give stronger indications of grave violations of international law as global outrage grew.

World Council of Churches acting general secretary Rev. Ioan Sauca, expressed abhorrence at the reported atrocities shown on global television channels and that were reported by the Associated Press and Reuters, calling for full investigation.

Pope Francis had the day before condemned the "sacrilegious war" in Ukraine at an open-air mass in Malta, ahead of a visit to a migrant centre preparing to take refugees fleeing Russia's invasion, AFP reported.

He spoke amid international outrage over the killing of civilians near the Ukrainian capital of Kyiv, after officials in Bucha said nearly 300 bodies had been found in mass graves after Russian forces withdrew.

Ukraine's foreign minister Dmytro Kuleba said the horrors seen Bucha, which sits 20 miles (32 kilometers) northwest of Kyiv, were "just the tip of the iceberg" during a news conference in Warsaw with Britian's Foreign Secretary Liz Truss on April 4.

He said: "Ukraine won the battle for Kyiv, but the war goes on. We are preparing for the new large-scale offensive by Russia in eastern Ukraine," Sky News reported.

"They will try to capture more territories in Donetsk and Luhansk region regions.

"They will try to entrench and root themselves in the Kherson region, they will try to capture bleeding Mariupol.

'TIP OF THE ICEBERG'

"What you've seen, the horrors that we've seen in Bucha, are just a tip of the iceberg of all the crimes that have been committed by the Russian army in Ukraine so far.

"And I can tell you that without exaggeration and with great sorrow that the situation in Mariupol is much worse compared to what we've seen in Bucha and other cities, and towns and villages nearby Kyiv."

A Ukrainian official described it as a "scene from a horror movie," after the reported massacre in Bucha, Global News reported.

World Council of Churches' Sauca said, "War is a context inherently conducive to such brutality, which underlines the need for systems of legal accountability to prosecute perpetrators, in order to curb the worst in humanity.

"Moreover, it underscores the urgent necessity of bringing this terrible conflict to an immediate end, for the sake of preventing yet more death, injury and destruction of communities."

Sauca added: "We appeal to those responsible for conceiving, pursuing and supporting this war, to stop the bloodshed and destruction, and to save the lives of all the children, women and men in the way of their ambition."

Russia's defense ministry said on April 3 that its forces did not kill civilians in Bucha after it was retaken by Ukrainian soldiers from Moscow's troops.

"During the time this settlement was under the control of Russian armed forces, not a single local resident suffered from any violent actions," the ministry said in a statement, saying Russia's military delivered 452 tons of humanitarian aid to civilians there.