Pope Francis celebrated Easter mass on Sunday with a message of social solidarity as the world's Orthodox marked a holiday marred by bitter divisions over Ukraine and clashes broke out in Jerusalem.

Francis led the ceremony for the holiest day in the Christian calendar on St Peter's Square, which will be followed by a special blessing and address to crowds of Catholic faithful in the famous Vatican piazza.

Pro-Kremlin rebels in east Ukraine appealed Sunday for Russian "peacekeepers" to sweep in after a deadly gunfight killed at least two of their militants, shattering an Easter truce and sparking "outrage" in Moscow.

But the Western-backed authorities in Kiev claimed the violence was a set-up by Russia to create a pretext for it to send troops in.

Politics overshadowed Easter observances in Kiev and Moscow, with the Orthodox religious leaders in each capital trading barbs over the crisis in Ukraine -- while the U.S. reportedly prepared to send ground forces to neighboring Poland.

Patriarch Filaret thundered to the faithful in pro-West Kiev that Russia was an "enemy" whose "attack" on Ukraine was doomed to failure because it was evil and contrary to God's will.

The newspaper's edition was set, ready to print, when the assailants came at 2 o'clock in the morning, Molotov cocktails in hand.

"We heard someone beating violently against our windows. They ripped off the shutters and broke the glass. Then they took two 1.5 litre (half gallon) bottles, filled with petrol, and they burnt the place."

Russia is an "enemy" whose attack on Ukraine is doomed to fail, Kiev's Orthodox patriarch thundered late Saturday in an unprecedented and fierce Easter broadside against Moscow.

"The country which gave us a guarantee of our territorial integrity has committed an attack. God cannot be on the side of evil, and that is why the enemy of the Ukrainian people is condemned to fail," Patriarch Filaret said in his pascal oration.

The head of the Russian Orthodox Church led prayers in Moscow for the Russians of Ukraine on Saturday, condemning those who he said wanted to "destroy Holy Russia".

"Ukraine is politically a foreign country (but) spiritually and historically that was never the case," Patriarch Kirill said before the start of observances on the eve of Orthodox Easter at Moscow's Christ the Saviour Cathedral.

A majority of inhabitants in Ukraine's pro-Russian protest hub Donetsk do not want to join Russia but consider the government in Kiev to be illegitimate, according to a poll published Saturday.

52.2 percent of people questioned in the region, the focal point of separatist unrest that has seen pro-Moscow militants seize a string of towns, said they were against joining Russia while 27.5 percent favored rule from Ukraine's former Soviet master Moscow.

President Vladimir Putin on Saturday said nothing should impede the normalization of relations between Russia and the West, after ties hit a post-Cold War low due to the standoff over Ukraine.

His remark, which contrasted with weeks of hostile rhetoric on both sides, came after talks between Russia, Ukraine and the West on Thursday in which an agreement was forged on initial steps to ease the crisis.

Russia was under intense U.S. pressure Saturday to convince pro-Moscow rebels in Ukraine to give up eastern towns they are occupying, after Washington said the situation in the former Soviet republic was in a "pivotal period".

But with the separatists' refusal to budge throwing a deal to defuse the crisis into doubt, and U.S. sanctions looming large, Russia was biting back -- warning that its military is massed on Ukraine's border, ready to act.

The White House warned Russia Friday that it was closely watching and evaluating whether Moscow was honoring a deal reached in Geneva designed to ease tensions in Ukraine.

National Security Advisor Susan Rice said Moscow must quickly rein in pro-Russia separatist groups in eastern Ukraine and allow Organization for Security and Cooperation in Europe (OSCE) monitors into the area.