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

The Russian assurance came as the Organisation for Security and Cooperation in Europe (OSCE) dispatched a negotiating team on Saturday to try to secure the release of its observers who are being held by in eastern Ukraine.

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"We believe that these people should be released as soon as possible," Russia's envoy to the OSCE, Andrei Kelin, told Russia's state RIA Novosti news agency.

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

Russian's foreign ministry also issued a statement saying Moscow was "taking measures" to resolve the situation but did not provide details.

'Spying mission'

Vyacheslav Ponomarev, the self-proclaimed people's mayor of Slovyansk and leader of a pro-Russian group, said the detained group could be released in exchange for jailed pro-Russian activists.

Ponomarev described the detained observers as "captives" and said that they were officers from NATO member states.

"As we found maps on them containing information about the location of our checkpoints, we get the impression that they are officers carrying out a certain spying mission," Ponomarev said.

The German-led, eight-member team was travelling under the auspices of the OSCE when they were detained on Friday.

Germany's Defence Ministry said it had had lost contact with the team, which it said also included five Ukrainians.

Al Jazeera's Barnaby Philips, reporting from Donetsk, said that a team was en route to negotiate the release of the observers.

"Some sort of OSCE mediation is on its way here to try to talk about this situation," he said.

The foreign observers 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.

Despite assurances from Russia that it would work to secure the release of the team, interim Ukrainian Prime Minister Arseniy Yatsenyuk said the seizure was "another proof and evidence that these so-called peaceful protesters with Russian ideas are terrorists."

'Provoke war'

In a briefing with reporters, he lashed out at Moscow, saying Russian military aircraft violated Ukrainian air space late on Friday.

"Russian military aircraft today overnight crossed and violated Ukrainian airspace seven times. The only reason is to provoke Ukraine to start a war," he said.

Russia's defence ministry, however, denied claims that its planes had made any such violation.

"Russia's airspace monitoring systems have not registered any violations of air borders of the states adjacent to Russia, including Ukraine," the defence ministry said in a statement carried by the state ITAR TASS news agency.

Meanwhile, Ukraine's acting defence minister Myhailo Koval said the Ukrainian military will fight if Russia sends in troops under the guise of a peace-keeping operation, the Interfax Ukraine news agency reported on Saturday.

"The United Nations has given no such (peace-keeping) mandate to the Russians. Everybody is already fed up with Russia's games with peace-keeping," the news agency quoted Koval as saying.

"If they come, they'll get what's coming to them: we will conduct combat operations."