Ukrainian President Petro Poroshenko accused the Kremlin Friday of direct involvement in the sniper deaths of anti-Russian protesters last year in Kyiv's Maidan Square.
He made the charge on the first anniversary of the killings, saying Ukraine's security services have the evidence.
Poroshenko told relatives of some of those killed that "just a few days ago, the head of state security told me that special forces operatives gave evidence that Russian presidential aide Vladislav Surkov led the organization of groups of foreign snipers on the Maidan."
The Russian foreign ministry called the accusation "ravings" and "madness."
The White House said again Friday that Moscow faces higher costs and more isolation if Russian-backed separatists continue violating the cease-fire in eastern Ukraine signed last week.
France's president Francois Hollande and Germany's chancellor Angela Merkel said Friday more sanctions are possible if there is no end to violations of the Minsk peace accord.
U.S. diplomats in Washington quietly endorsed the Europeans' view, saying there would be "additional costs" for those who ignore the Minsk agreement.
Speaking at a joint news conference, ahead of next Tuesday's meeting of French, German and Ukrainian foreign ministers in Paris, Hollande and Merkel said they were working hard to ensure the latest agreement between Russia and Ukraine is respected.
The two leaders met as the Ukrainian government and separatist rebels traded accusations Friday of violating a cease-fire agreement that was supposed to begin last Sunday.
Hollande was asked if he could confirm reports of Russian tanks entering Ukraine. He said he had no imemdiate confirmation.
That does not mean it did not happen, he said, but that he had not been formally notified. He said there is a risk of escalating violence, and that is all the more reason for both sides to fully respect the cease-fire agreement and withdraw heavy weapons.
Hollande also said those who violate cease-fire terms agreed in Minsk risk further sanctions. He added that the goal is not more sanctions, but rather securing peace in Ukraine.
Merkel agreed it is vital to respect the peace deal agreed to in Minsk. If not, she also suggested the European Union may re-examine the sanctions question.
Earlier this week, the EU expanded its sanctions against Russia, adding new names to a list of people facing restrictive measures.
A British parliamentary report published Friday has sharply criticized the European response to the Ukrainian crisis.
Meanwhile, the U.N. Human Rights Office expressed grave concern at the fighting in eastern Ukraine, which it says is taking a heavy toll on civilians caught in the line of fire. The agency also accused pro-Russian rebels of preventing European monitors access to Debaltseve, seized by the separatists in the last few days, despite a cease-fire.
The U.N. Human Rights Office reports nearly 5,700 people have been killed and over 14,000 wounded since mid-April of last year.
While these numbers are dramatic, Human Rights spokesman Rupert Colville says they represent a very conservative estimate.
"The Human Rights monitoring unit in Ukraine, our team and WHO who jointly gather these statistics believe the actual number may be considerably higher. And, a further increase in those figures is anticipated in the coming days because the reporting of the casualties, especially military casualties - both during the pre-cease-fire period and especially in recent days in Debaltseve have been considerably delayed and we do not think we have anything like the full picture of casualties from Debaltseve," said Coleville.
He says his agency remains deeply worried about the fate of civilians and captured or wounded Ukrainian servicemen in the Debaltseve area who were caught in the fighting.
The U.N. Human Rights Office is issuing a separate appeal on behalf of Ukrainian pilot Nadiya Savchenko, who has been detained in Moscow since last July. It notes that she has been on a hunger strike for 70 days and that her condition is deteriorating. It is calling on Russian authorities to release her on humanitarian grounds.
Lisa Schlein contributed to this report from Geneva.