An insurgent leader's fighters in Ukraine are holding four observers from the Organisation for Security and Cooperation in Europe.

The military observers from the Organisation for Security and Cooperation in Europe who were held for more than a week have been released, the leader of pro-Russia insurgents in the eastern Ukrainian city of Slovyansk has said.

Gunfire and blasts have been heard around an eastern city in Ukraine that has become the focus of an armed pro-Russian insurgency.