(Recasts with U.N. stating Russian troops cannot be in UNDOF)

UNITED NATIONS/MOSCOW, June 7 (Reuters) - The United Nations on Friday thanked Russia for offering to replace peacekeepers from Austria in the Golan Heights but said an agreement between Israel and Syria bars all permanent members of the Security Council from the U.N. observer mission there.

President Vladimir Putin made the offer in Russia on Friday after Vienna said it would recall its troops from a U.N. monitoring force due to worsening fighting in Syria.

Austria, whose peacekeepers account for about 380 of the 1,000-member U.N. force observing a 4-decade-old ceasefire between Syria and Israel, said it would pull out after intense clashes between Syrian government forces and rebels on the border.

But U.N. spokesman Martin Nesirky said it was impossible for the United Nations to accept the offer from Russia, which along with the United States, Britain, France and China, is a permanent veto-wielding member of the 15-nation Security Council.

"We appreciate the consideration that the Russian Federation has given to provide troops to the Golan," he told reporters. "However, the Disengagement Agreement and its protocol, which is between Syria and Israel, do not allow for the participation of permanent members of the Security Council in UNDOF."

The departure of the Austrians from the United Nations Disengagement Observer Force is the latest blow to monitoring force. In addition to the increased fighting in its zone of operation, there have been several recent incidents in which Syrian rebel forces detained UNDOF monitors.

Since 1974 UNDOF has had the task of monitoring the "area of separation," between Syrian and Israeli forces, a narrow strip of land running 45 miles from Mount Hermon on the Lebanese border to the Yarmouk River frontier with Jordan. The force has helped keep the area relatively stable, U.N. diplomats say.

Russia, a longtime ally and arms supplier to Syrian President Bashar al-Assad, has been trying along with Western powers to bring the warring sides in Syria together into talks on a solution to the more than 2-year-old conflict. A planned Geneva peace conference has been delayed until July at least.

The U.N. Security Council will meet later on Friday to discuss the Austrian withdrawal after anti-Assad rebels briefly seized the crossing between Israel and Syria, sending U.N. staff scurrying to bunkers before Syrian soldiers managed to push them back.

Nesirky said a meeting was under way between U.N. peacekeeping officials and troop contributing countries to find a member state willing to offer monitors to replace the departing Austrians, who make up a third of the force. (Reporting by Alissa de Carbonnel in Moscow and Louis Charbonneau in New York, Editing by Thomas Grove/Mark Heinrich)

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