French President Francois Hollande on Friday paid tribute to the "courage of the Red Army" and the "decisive contribution" of the former Soviet Union in winning World War II.

Hollande was addressing a score of world leaders and hundreds of D-Day veterans on the 70th anniversary of the landings, which this year has shared the limelight with a crisis in Ukraine that has pushed the West's relations with Russia to Cold War lows.

Russian President Vladimir Putin will pay a working visit to Austria on June 24, where he will meet with leaders to discuss the situation in Ukraine, official sources in Vienna said Friday. 

Putin was to hold talks with President Heinz Fischer and Chancellor Werner Faymann during the one-day flying visit, their respective offices told Agence France Presse. 

British Prime Minister David Cameron on Thursday told his Russian counterpart Vladimir Putin that Moscow must recognize the authority of the new president-elect in Ukraine.

Delivering what he said was a "very clear and firm" message from the West, Cameron said: "Russia needs to properly recognize and work with this new president," in comments to the BBC.

Ukraine's president-elect Petro Poroshenko said Friday's D-Day commemorations would serve as a show of European unity and support for Kiev in its current crisis.

On a visit to Berlin for talks with Chancellor Angela Merkel Thursday before heading to France to mark the Allied landing in June 1944, Poroshenko said he was grateful for Western backing.

U.S. President Barack Obama on Thursday said he had "expressed some concerns" to France about its sale of warships to Russia.

"I have expressed some concerns, and I don't think I am alone," Obama said at a news conference in reply to a question about France's controversial decision to go ahead with a deal to sell two Mistral warships to Russia despite events in Ukraine.

Ukrainian authorities said Thursday they had closed three checkpoints on the border with Russia after nightly assaults by separatists.

The move came as the government vowed to beef up its security presence to counter pro-Russian rebels, amid reports of continued fighting in the country's east.

D-Day veterans marched back to Normandy's beaches and villages on Thursday, in an emotional return marking 70 years since the launch of the biggest amphibious invasion in military history.

Royals, top brass and no fewer than 20 world leaders, including U.S. President Barack Obama and Russia's Vladimir Putin, will attend the main D-Day ceremony on Friday, amid ongoing diplomatic wrangling over the Ukraine crisis.

Syria's exiled opposition vowed Thursday to continue its uprising against President Bashar Assad, saying his election to a new seven-year term was "illegal".

"The Syrian National Coalition reaffirms that this election is illegal and does not represent the Syrian people," it said in response to Assad's landslide victory in a Tuesday election held only in government-controlled areas.

The head of the mission overseeing the destruction of Syria's chemical arsenal urged Damascus on Wednesday to urgently hand over its remaining agents, and pressed countries with influence to intervene.

Under a deal backed by the United Nations and brokered by the United States and Russia after Washington threatened air strikes against Syrian government targets, the weapons were to be destroyed by June 30.

Two groups of international observers who have gone missing in the east of Ukraine are likely being held in the restive region of Lugansk, a separatist leader told Agence France Presse Wednesday.

The two teams from the Vienna-based Organization for Security and Cooperation in Europe (OSCE) -- totaling eight monitors and one local translator -- have been unaccounted for since last week.