OSCE chairman Didier Burkhalter and German Foreign Minister Frank-Walter Steinmeier have called on pro-Russian separatists to release European military observers held in eastern Ukraine. Burkhalter and Steinmeier met in Bern, Switzerland, to discuss the fate of OSCE observers held in the city Sloviansk since April 25, dpa reported. They also considered the option of strengthening the OSCE mission to the country. The fate of the observers is also on the agenda of talks between Steinmeier and EU foreign policy chief Catherine Ashton in Berlin on Friday.