Russia pledged Saturday to help free a group of international OSCE observers held hostage by pro-Kremlin rebels in eastern Ukraine who accuse them of being "NATO spies".

"We believe that these people should be released as soon as possible," Russia's envoy to the Organisation for Security and Cooperation in Europe, Andrei Kelin, told Russia's state RIA Novosti news agency.

"As an OSCE member, Russia will take all possible steps in this case."

Russia's foreign ministry said Moscow was "taking measures" to resolve the situation but did not provide details.

Ukraine's SBU security service meanwhile said the monitors were being held "in inhuman conditions".

They said one of the group "urgently" needed medical help but the militants had refused.

"The terrorists are planning to use the hostages as a human shield, effectively terrorising the whole international community and all members of the OSCE," the SBU said.

The rebels seized the 13 members of a military observer mission belonging to the pan-European security body on Friday and have refused to set them free.

They were sent to Ukraine to monitor an April 17 accord signed in Geneva between Russia, Ukraine, the United States and the European Union that was meant to de-escalate the dangerous crisis in the ex-Soviet republic.

The OSCE personnel were being detained in the flashpoint town of Slavyansk, which insurgents seized two weeks ago and which is now under siege by the Ukrainian military.

"We arrested some NATO spies... they will be exchanged for our own prisoners. I don't see any other way they will be freed," Denis Pushilin, the head of the insurgents' self-declared Donetsk Republic, told reporters on Saturday.

Slavyansk's self-styled mayor, Vyacheslav Ponomaryov, also told reporters that they would not be released "until we verify who they are and what aim they came with".

"They are of course prisoners. They are fine," he added.

"They have nothing to do with the OSCE. They are military personnel," he claimed.

He also said they should have asked for "permission" from the rebels to travel in the area.

- 'Not in Ukraine to intervene' -

Late Friday, German Defence Minister Ursula von der Leyen confirmed that pro-Russian separatists had arrested the 13 mission members, operating under German command.

Four are Germans, including three members of the German military.

An OSCE spokeswoman at the group's Vienna headquarters said there were four other non-German members of the organisation being held: a Dane, a Pole, a Swede and a Czech.

This group of eight monitors was accompanied by five Ukrainian army personnel, the defence ministry in Kiev said in a statement.

The group's detention sparked a chorus of protest worldwide with calls for their immediate release.

A special diplomatic crisis cell set up in Berlin is working "with all its might" to resolve the situation, von der Leyen's office said.

"The observers and our soldiers are not in Ukraine to intervene in any way," she stressed in a statement.

"They have the important task of confidence building and improving transparency."

German Foreign Minister Frank-Walter Steinmeier called his Russian counterpart Sergei Lavrov to demand Moscow use its influence to secure the group's freedom.

"We are using all the means at our disposal to... ensure that they can continue their mission," the foreign ministry said in a statement in Berlin.

Czech Prime Minister Bohuslav Sobotka said the separatists' actions "are not helping calm the situation" but rather "unfortunately contribute to its escalation".

Ukraine, the United States and many EU countries believe Russia is in control of the rebels and fomenting unrest in Ukraine to keep it under its post-Soviet influence.

But the Russian foreign ministry later issued a statement appearing to lay the blame for the monitors' detention at Ukraine's feet.

"They were invited by the Ukrainian authorities" and Kiev is responsible for their safety, the ministry said.

Moscow suggested Kiev's authorities should have agreed the scope of the OSCE's work in regions where they "do not control the situation, and where a military operation against residents of their own country is taking place".

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