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* OSCE countries invited to send observers to Crimea
* Week-long mission aims to defuse military tensions
* U.S. confirms to send two observers
KIEV/VIENNA, March 4 (Reuters) - Ukraine said on Tuesday that observers from a pan-European security body would travel at its invitation to the Crimea region, where Russian forces have taken control, in an attempt to defuse a military standoff.
"An OSCE mission has arrived in Kiev which will go to the Crimean peninsula to monitor the situation," Ukraine's national security chief, Andriy Paruby, told a news conference in Kiev.
He said the security situation on the Black Sea peninsula was "complicated but stable".
Several members of the Organization for Security and Cooperation in Europe were set to send up a maximum of two observers each on the mission, diplomatic sources at OSCE headquarters in Vienna said.
The United States has agreed to take part, U.S. envoy Daniel Baer said.
"The invitation is for a week-long visit initially. The United States will dispatch two monitors per the request of the Ukrainian government," he told Reuters.
He said around 15 observers had been volunteered so far and more responses were still coming in from OSCE members.
It was not immediately clear whether Russia would allow monitors to enter the Black Sea peninsula, where it controls the airspace and access points. The diplomats said Russia's agreement was not legally necessary.
The Russian delegation to the OSCE did not respond to an emailed request for comment.
President Vladimir Putin ordered troops involved in a military exercise in western Russia back to base on Tuesday and called any use off force in Ukraine a last resort, helping to ease East-West tension over fears of war.
But forces loyal to Moscow remained in control of Crimea, seized bloodlessly after Russian ally Viktor Yanukovich was ousted as Ukrainian president last month, and are surrounding military compounds of the Ukrainian army and navy.
A military observer operation would not need a special OSCE mandate because it is already covered by the so-called Vienna Document on confidence building measures and risk reduction.
The OSCE, a pan-European forum for security issues that was set up during the Cold War, is also trying to put together a "contact group" of leading players in the Ukraine drama and to arrange a broader monitoring mission for Ukraine.
Those monitors could take stock of human rights, assess treatment of minority populations and evaluate security concerns raised by both sides to the conflict in Ukraine.
But such a mission would require a consensus decision, meaning Russia's support would be needed. Russia's OSCE envoy had been non-committal on the idea on Monday, telling reporters more details were needed. (Reporting by Pavel Polityuk in Kiev and Michael Shields and Fredrik Dahl in Vienna; Editing by Paul Taylor)