Ukraine has accused Moscow of interfering in restive eastern cities in a bid to restore the Soviet Union. The warning was echoed by Ukrainian religious leaders who traded accusations with their Russian counterparts.
In an interview with NBC's "Meet the Press," Ukrainian Prime Minister Arseniy Yatsenyuk has warned that Russian President Vladimir Putin dreamed of restoring Moscow's former geopolitical and territorial power.
According to excerpts of the interview released ahead of Sunday's broadcast, Yatsenyuk said: "President Putin has a dream to restore the Soviet Union. And every day, he goes further and further. And God knows where is the final destination."
The Ukraine and the West have accused Russia of orchestrating unrest in the east in order to justify a military intervention similar to the one seen in the Crimean Peninsula.
Pro-Russian activists remain in control of government buildings in more than a dozen eastern cities.
They have rejected the terms of Thursday's Geneva deal, in which Russia, the US, the EU and Ukraine agreed that activists must lay down their arms and end their occupation of administrative offices.
The separatists insist they will remain where they are until the "illegitimate" government in Kyiv - established after the fall of pro-Kremlin president Viktor Yanukovych - steps down. They have also reiterated calls for a referendum on joining the Russian Federation.
In a bid to calm the crisis, the deputy head of the Organization for Security and Cooperation in Europe's (OSCE) mission in Ukraine, Mark Etherington, is to begin negotiations with separatists in the eastern city of Donetsk later on Sunday.
The Ukrainian Foreign Ministry has also said it is suspending its "anti-terrorist operation" against pro-Russian activists for the Easter period.
Nevertheless, proof that tensions remain high came from religious leaders on opposing sides of the conflict, who delivered deeply political Easter Sunday messages.
The head of Ukraine's Orthodox Church condemned Russian "aggression" and said "evil" would be defeated.
"God cannot be on the side of evil, so the enemy of the Ukrainian people is condemned to defeat," Patriarch Filaret told worshippers in Western-friendly Kyiv.
Across the border in Moscow, Russian Church Patriarch Kirill, called on God to put "an end to the designs of those who want to destroy Holy Russia".
He said that while Ukraine was "politically" separate, "spiritually and historically" it was at one with Russia, and he prayed that it would benefit from authorities that are "legitimately elected".
"We are a single people before God," he added.
US 'readying troops for Poland'
Allegations of Moscow's alleged interference in eastern Ukraine have been compounded by reports that more than 40,000 Russian troops are massed along Ukraine's eastern border.
On Saturday the Washington Post reported that the United States was responding to the perceived threat by working to build up NATO's military presence in the region.
It said it was preparing to deploy ground troops to Poland, a NATO member, and was also considering sending troops to several Baltic states.
The US is seeking to push Moscow to use its influence in eastern Ukraine to realize the terms of the Geneva deal.
It has been joined by the EU in levelling a number of sanctions against Russian and Ukrainian individuals.
The US has also warned of further, tougher penalties targeting the Russian economy - a threat which has not won unequivocal support in Europe, particularly among countries who rely on Russian gas.
Germany cautious over sanctions threat
In an interview with the Bild newspaper on Sunday, German foreign minister Frank-Walter Steinmeier said the focus must be on deescalating the situation, rather than implementing further economic sanctions.
"I sometimes wish that the same engagement being used for the debate about sanctions would also exist when it comes to avoiding a further escalation," Steinmeier told the Sunday newspaper, according to excerpts released before publication. "We've already exhaustively discussed the sanctions issue," he added.
Russia has fiercely denied that allegations that it plans to invade and has accused the US of treating Russian "like a naughty schoolboy."
It has, however, defended its right to protect eastern Ukraine's large ethnic Russian population.
ccp/jm (AFP, Reuters, dpa)