President Barack Obama met Ukraine's president-elect Petro Poroshenko on Wednesday, in a show of U.S. support for Ukraine's right to chart its own future, before an encounter with Russia's Vladimir Putin.

Obama sat down with Poroshenko in Warsaw, during a trip designed to assuage security concerns in eastern Europe following Russia's annexation of Crimea and what Washington says is an effort to destabilize Ukraine. 

Russian President Vladimir Putin will meet German Chancellor Angela Merkel on Friday in France, where they are attending D-Day anniversary events, a Kremlin aide said Tuesday.

"Before the ceremonial events, on Friday morning a meeting is planned with German Chancellor Angela Merkel," said presidential aide Yury Ushakov, cited by the RIA Novosti news agency.

Russian President Vladimir Putin will hold face-to-face talks on the Ukraine crisis with British Prime Minister David Cameron, Downing Street said on Monday.

The two leaders will meet after they attend events to mark the 70th anniversary of the D-Day landings in Normandy, France, on Friday.

U.S. lawmakers urged France to break its contract to sell two warships to Russia and instead sell or lease them to NATO, which said Friday it was up to Paris to decide.

Three congressmen led by Eliot Engel, the top Democrat on the House Foreign Affairs Committee, wrote to the transatlantic alliance's Secretary General Anders Fogh Rasmussen expressing concern about the construction and sale to the Russian navy of two Mistral-class helicopter carriers.

A team of international monitors with the OSCE was detained at a checkpoint in restive eastern Ukraine on a day of deadly fighting between government forces and pro-Russian gunmen, officials said Tuesday.

The Organization for Security and Cooperation in Europe said it had lost contact with the four-member team as they were on patrol in the main rebel-held city of Donetsk on Monday.

Russia said Monday it was ready to talk to Ukraine's new president Petro Poroshenko, raising hopes that after months of tensions Moscow sees in the billionaire a leader it can work with.

"We are ready for dialogue with Kiev's representatives, with Petro Poroshenko," Foreign Minister Sergei Lavrov said in Russia's first reaction to his victory in Sunday's vote.

Russian President Vladimir Putin on Saturday condemned remarks from Britain's Prince Charles reportedly comparing his actions to those of Adolf Hitler, saying they were not worthy of royalty.

"I did not hear this expression. If it was said then of course this is unacceptable," Putin told foreign news agency journalists in Saint Petersburg.

Russian President Vladimir Putin said Saturday that it was "impossible" to isolate Russia from the global economy despite pressure from the West over the crisis in Ukraine.

He also rejected talk of a "new Cold War" between Moscow and the West and denied claims Russia was trying to revive the Soviet empire.

Syrian President Bashar Assad Saturday thanked key ally Moscow for its support during a meeting in Damascus with a Russian delegation led by deputy premier Dimitry Rogozin, state media said.

The meeting comes days after Russia vetoed a draft U.N. resolution that would have referred crimes committed in Syria to the International Criminal Court (ICC).

Russian President Vladimir Putin pledged Friday to respect the outcome of Ukraine's presidential election but warned that the country had descended into all-out civil war after a bloody upsurge in separatist violence.

At least seven people were killed in fighting between rebels and defense forces outside the eastern industrial hub of Donetsk on Friday, a day after the deaths of 18 soldiers in the heaviest loss for the Ukraine military since the conflict began.