Germany has welcomed a Russian offer to help efforts to free a group of European military observers in eastern Ukraine where they are being held by pro-Russian forces.

Russia’s Foreign Minister Sergei Lavrov has spoken by phone on Saturday to Swiss President Didier Burkhalter, acting president of the Organisation for Security and Cooperation in Europe (OSCE).

A separatist leader in Slovyansk has offered to exchange the eight German-led monitors for fellow rebels being held by the Kyiv authorities. The de facto mayor of the town, Vyacheslav Ponomaryov, said the captives were being kept in good conditions.

“One of the soldiers suffers from diabetes, but it is not a serious condition, he is on tablets. There is medicine there is food,” said Ponomaryov.

He said the Russian government had not been in contact with him to discuss the observers’ release.

“They are not observers, they are spies. We have information confirming that. The information confirming their spying activities was in their papers,” said separatists’ representative Yevgeny Gorbik, echoing earlier claims.

Moscow has said what it calls “public structures” controlling parts of southeastern Ukraine had not been properly informed of the observers’ plans to travel there.

An OSCE negotiating team is said to be on its way to the region to try to secure the captives’ release.

Ukraine’s acting prime minister – who met Pope Francis in the Vatican on Saturday – has claimed that Russian military planes violated Ukrainian airspace.

Arseniy Yatsenyuk’s comments came after a visit to a church popular with his country’s Catholics, where he lit a candle for those killed in Ukraine’s unrest.

Kyiv has rejected Russian demands that it stop its military operations in the southeast to defuse the crisis.

EU diplomats are to meet on Monday to discuss further sanctions against Russia.