Francois Hollande and German Chancellor Angela Merkel lamented the "insufficient" progress in resolving the Ukraine crisis after a telephone conversation Monday with Russian President Vladimir Putin, according to a source close to the French president.

"The chancellor and the president (Hollande) raised the need to put pressure on the various parties" given that progress is "insufficient," the source said, a day before foreign ministers from Ukraine, Russia, France and Germany hold talks in Paris on the crisis.

U.S. Secretary of Defense Ashton Carter on a visit to Berlin Monday touted Washington's bid to station heavy military equipment in eastern Europe as an "important" move to help counter any Russian threat.

Amid mounting fears about Russia's actions and intentions in the former Soviet bloc, Washington has announced it is poised to move heavy equipment to the region to support training efforts, prompting Moscow to hit back with a threat to boost its own nuclear arsenal.

EU foreign ministers formally agreed Monday to prolong to January 2016 damaging economic sanctions against Russia to ensure it fully implements Ukraine peace accords, officials said.

"EU has extended economic sanctions against Russia until 31 January 2016, with a view to complete implementation of (the) Minsk agreement," an EU spokeswoman said in a tweeted message.

U.S. Secretary of Defense Ashton Carter will visit his European counterparts this week to take stock of cooperation with NATO in the wake of Russian aggression in Crimea and Ukraine.

The visit comes as officials say the military bloc's activity in Europe is at its strongest in decades.

Moscow threatened Friday to retaliate against state-linked foreign firms operating in Russia after its official assets in Western Europe were frozen over legal claims by former Yukos oil company shareholders.

Russian officials said state accounts had been frozen in Belgium, and representatives of claimants from the defunct oil firm said Russian assets were also blocked in France.

Russian President Vladimir Putin said Friday he is ready to "push" his Syrian counterpart Bashar Assad towards introducing reforms in the war-torn country, while vowing to continue to support his ally.

"We are ready to work with Assad so that he engages in a process of political change," Putin said at the Saint Petersburg International Economic Forum.

Russia's chief negotiator was cautiously upbeat Friday about  nuclear talks between Iran and six world powers, saying the number of issues still to be resolved before a June 30 deadline "can be counted on one hand."

"Some complex issues remain but there are fewer of them than a week ago," Russian news agency RIA-Novosti quoted negotiator Sergei Ryabkov as saying at the latest round of talks in Vienna.

Vladimir Putin told foreign investors Friday his country would cooperate with the West despite persistent tensions as Russia and Greece agreed to build a gas pipeline amid a debt crisis in Athens.

Speaking at Russia's premier economic forum against a backdrop of tensions over the Ukraine crisis, the Russian president insisted Moscow was "open to the world."

The EU on Friday prolonged until 2016 a sanctions package over Russia's illegal annexation of Crimea, days after rolling over a series of damaging punitive measures against Moscow for its role in the Ukraine crisis.

The European Council said the 28-nation bloc continues to "condemn the illegal annexation of Crimea and Sevastopol by the Russian Federation and remains committed to fully implement its non-recognition policy."

Russian Deputy Foreign Minister Mikhail Bogdanov has repeatedly said that Free Patriotic Movement leader MP Michel Aoun should give up his candidacy for the presidency to resolve Lebanon's political crisis.

Lebanon's Ambassador to Moscow, Shawqi Bou Nassar, has said in a report that Bogdanov was in favor for Aoun to “pave way for other candidates and allow other Christian personalities to announce their candidacies.”