MOSCOW—Ukraine suffered heavy losses in a counterattack along a key supply route to the separatist capital of Donetsk, officials said Thursday, underscoring the significant threat posed by pro-Russia rebel fighters despite steady advances by government forces.

Losses have mounted recently, adding more pressure on Kiev to seek a compromise at peace talks next week in the Belarusian capital of Minsk, where Ukrainian PresidentPetro Poroshenko and Russian leader Vladimir Putinare scheduled to meet face-to-face for the first time in two months.

"Ukraine is seeking peace," Mr. Poroshenko said of the planned meeting on a visit to the eastern town of Mykolayiv on Thursday. "The whole world is tired of war."

Officials say Mr. Poroshenko hopes to get backing for his peace plan, which calls for rebels in eastern Ukraine putting down their weapons in return for possible amnesty and more autonomy for the country's regions. The insurgents say they have no intention of capitulating.

Russian Foreign Minister Sergei Lavrov spoke by telephone with his German counterpart Frank-Walter Steinmeier, discussing the need for a cease-fire and the start of a "national dialogue" in Ukraine, according to a Russian government statement.