German Chancellor Angela Merkel said on Monday that a ceasefire agreement in eastern Ukraine was not being respected and that everything possible must be done to fully implement a peace deal reached in Minsk, Belarus, in February.
"We are here to implement the Minsk deal, and not to call it into question," Merkel said at a news conference in Berlin with French President François Hollande and Ukrainian President Petro Poroshenko.
Earlier on Monday, Poroshenko said Ukraine was facing a precarious year struggling to fend off what he called Russian strategies to undermine Kyiv's drive for closer relations with Europe.
At a ceremony in central Kyiv to mark 24 years of Ukrainian independence from Moscow, he said 50,000 Russian troops were massed along the Ukraine-Russia border and a further 9,000 were supporting pro-Russian rebels in separatist territories.
"The war for Ukrainian independence is continuing," Poroshenko said.
While the threat of "full-scale military invasion" remains, Russia "also has an alternative strategy — to undermine the situation in the middle of our country and to put the country at loggerheads with key [foreign] partners, isolating it with the aggressor," he said.
Later in Berlin, Poroshenko said Ukraine was fulfilling its obligations in the peace deal.
"We clearly declare that today Russia and the fighters it supports are the only threat to the peace process," he said.
Russian Foreign Minister Sergei Lavrov called Poroshenko's comments "unfounded and disgraceful," and said he was trying to break the "genetic code" ensuring the unity of the two nations.
Lavrov said Russia, which was not present at the talks in Berlin, wanted Germany and France to put pressure on Poroshenko to stick to Ukraine's commitments under the Minsk agreement and to grant the rebel regions greater autonomy.
Russia annexed Ukraine's Crimea peninsula in 2014 after an uprising toppled a pro-Russian president in Kyiv. It has dismissed accusations by NATO and other powers that it has sent arms and troops to back separatists in the east.
Under the Minsk agreement, both sides have withdrawn large numbers of heavy weapons from the conflict zone but sporadic clashes have still proven deadly.
More than 6,500 people, including hundreds of civilians, have been killed since fighting erupted.