Russia and Ukraine appeared to have made a long-awaited breakthrough in efforts to resolve a damaging crisis in their relations after conciliatory talks on the sidelines of Friday's D-Day anniversary ceremonies.

Vladimir Putin and Ukrainian president-elect Petro Poroshenko embraced the spirit of the day that signaled the end of World War II by announcing they would jointly seek a ceasefire in the conflict between government forces and pro-Russian rebels in southeastern Ukraine.

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.

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.

World leaders urged Vladimir Putin on Wednesday to stop destabilizing Ukraine or face further sanctions as they met without a Russian president for the first time since the 1990s.

Putin reached out a hand despite being banned from the Group of Seven summit following Russia's annexation of Crimea in March, saying that he was ready to meet Ukraine's president-elect.

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.

German Chancellor Angela Merkel reiterated a warning Wednesday ahead of a G7 summit that Russia could face new stepped-up economic sanctions if it does not help defuse the crisis in Ukraine.

Hours before the Group of Seven rich countries begins talks in Brussels without Russia, Merkel called in a speech to parliament for stiffer border controls by Russia to stop fighters or weapons crossing into Ukraine.

Three Ukrainian soldiers have been injured in a massive all-night attack carried out by hundreds of pro-Russian insurgents in the nation's restive east, authorities in Kiev said Wednesday.

The assault on a position held by the Ukrainian National Guard in the Lugansk region began Tuesday evening and lasted 10 hours, the interior ministry said, adding that six rebels were also killed in the fighting.

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.