Russia's President Vladimir Putin (3rd L) shakes hands with his Ukrainian counterpart Petro Poroshenko, with Belarus' President Alexander Lukashenko (L), France's President Francois Hollande (2nd L) and Germany's Chancellor Angela Merkel standing nearby, as they take part in peace talks on resolving the Ukrainian crisis in Minsk, February 11, 2015. [Photo/Agencies]

The four leaders met alone at about 17:15 GMT and then went into a full summit with their delegations.

Earlier, Poroshenko said that without a de-escalation of the conflict and a ceasefire the situation would get "out of control". 

The outcome of the talks is expected to influence discussions at an EU summit in Brussels on Thursday, when sanctions against Moscow will be on the agenda.

Still, Moscow expressed optimism. A Russian diplomatic source said it was 70 percent likely that an agreement would be reached.

Foreign Minister Sergei Lavrov said there had been progress in the run-up to the summit but Kiev could be holding back a deal by insisting on control of the Russian-Ukrainian border, part of which is held by the separatists.

The talks are taking place while an International Monetary Fund mission is trying to negotiate a bailout to save Ukraine from bankruptcy.

Prime Minister Arseny Yatseniuk said he hoped for a deal in the next 48 hours and IMF chief Christine Lagarde said she would make a statement on Ukraine early on Thursday.