(Adds Steinmeier quotes)
BERLIN, April 28 (Reuters) - Germany urged Moscow on Monday to use its influence on pro-Russian separatists in eastern Ukraine to secure the release of European military observers being held in the city of Slaviansk.
Chancellor Angela Merkel's spokesman Steffen Seibert said Berlin condemned the detention of the team from the Organisation for Security and Cooperation in Europe (OSCE), including four Germans, calling it "against the law and without justification".
The release of one of the eight hostages on medical grounds was "a positive step" but the other seven must also be freed "immediately, unconditionally and unharmed", said Seibert.
"We ask the Russian government to act publicly and internally for their release, to distance itself clearly from such acts and to use its influence on pro-Russian perpetrators and forces in eastern Ukraine to secure their release," he said.
Seibert said the presence of armed masked men when the OSCE observers were paraded before the media in Slaviansk on Sunday - in what he called a "revolting spectacle" - made it clear beyond any doubt that they were the hostages of the city's self-styled mayor, Vyacheslav Ponomaryov.
German Foreign Minister Frank-Walker Steinmeier spoke with his Russian counterpart Sergei Lavrov on Saturday and again on Monday to urge Moscow to make it clear to the separatists that their action was unacceptable, a ministry spokesman said.
Speaking in Oslo late on Monday, Steinmeier said: "It is Russia's job to free the hostages. I have just had the leader of the OSCE on the phone. Unfortunately nothing has come out of the talks."
"My Russian colleague has told me by phone that Russian diplomacy is still backing the deal in Geneva and has encouraged all sides to lay down weapons," he added. "Russia should use all its influence to calm the situation down."
He said the talks would continue on Tuesday morning. (Reporting by Stephen Brown and Erik Kirschbaum in Berlin and Gwladys Fouche in Oslo; Editing by Tom Heneghan)