MINSK, Belarus

Ukrainian President Petro Poroshenko said Thursday that Russia set "unacceptable" conditions during the ongoing Minsk talks, but he did not give further details.

Talks between Poroshenko, Russia's Vladimir Putin, France’s Francois Hollande and Germany's Angela Merkel continued for 13 hours in the Belarusian capital in an effort to end bitter fighting in eastern Ukraine. 

Earlier on Thursday, Russian Foreign Minister Sergei Lavrov said there had been some progress.

"Everything is super," he said.

Heidi Tagliavini, Organization for Security and Co-operation in Europe (OSCE) special representative, joined the talks after Poroshenko's statement.

The leaders are expected to sign an agreement after the talks as they work to find a political solution to a conflict that has claimed the lives of 5,300 people and left 12,200 others injured. 

The Ukrainian government and pro-Russian separatists had signed a cease-fire agreement in Minsk on Sept. 5, 2014, but both sides have repeatedly violated it.

Fighting has increased in recent weeks as separatists, backed by Russia, seek to expand their control in the east of the country.

A press conference is scheduled for later this morning.