* U.S. wants Moscow to back observer mission to Ukraine
* Russian envoy to OSCE is non-committal
* Ukraine ambassador says Kiev will defend itself with "all possible means" (Adds quotes and background)
By Michael Shields and Georgina Prodhan
VIENNA, March 3 (Reuters) - A senior U.S. diplomat urged Russia on Monday to support sending an OSCE observer mission to Ukraine to help monitor human rights and defuse the tense military situation.
But Moscow's ambassador to the Organization for Security and Cooperation in Europe (OSCE), Europe's leading human rights and democracy watchdog, was non-committal, saying some international missions may do more harm than good.
Washington has been pushing for the United Nations or OSCE to send a mission of perhaps hundreds of people to take stock of human rights, monitor treatment of minority populations and evaluate security concerns raised by both sides of the conflict in Ukraine.
Such a mission would require a consensus decision, meaning Russia's support would be needed.
The U.S. Assistant Secretary of State for European and Eurasian Affairs, Victoria Nuland, lobbied for the move at a meeting of the OSCE permanent council in Vienna on Monday.
"There will be very, very broad consensus for that monitoring mission. We call on Russia to join that consensus, make the right choice and pull back its forces," she told reporters.
"We are using all of our channels of dialogue to make the case to Russia that it doesn't have to be this way, that it should make a 21st century choice to settle its issues politically and through negotiation, not with military force."
The OSCE, originally set up during the Cold War as a platform for dialogue between East and West, has played a significant role in monitoring human rights and other issues in many places in the past - for example in the countries that emerged from the bloody collapse of Yugoslavia in the 1990s.
But diplomats say its overall work in recent years has been hobbled by renewed East-West tension.
Russia's OSCE envoy, Andrey Kelin, did not rule a mission out but said Moscow needed more information on the plan, which the OSCE said could accompany a "contact group" it hoped to form of leading players in the drama.
"We need actually to have more details before making a decision," he told reporters.
"We know that there have been different missions. Some of them have done good for the countries which were in conflicts. But some of them, for instance in Kosovo, they haven't done good, but I should say more they have contributed to worsen the situation."
The OSCE is sending around three conflict prevention and police experts to Ukraine in the next 24 hours under existing mandates. It will also deploy a mission to observe presidential elections in Ukraine scheduled for May 25.
Ukraine's ambassador to the Vienna-based OSCE had told the council meeting that Kiev will use "all possible means" to defend itself from a Russian military threat,
"Ukraine is a peaceful country but it will protect its sovereignty and territorial integrity from an aggressive war with all possible means in full compliance with the UN charter," Ambassador Ihor Prokopchuk told a session of the human rights and democracy watchdog's permanent council.
He said Kiev wanted an immediate repeal of Russian authorisation to use force in Ukraine, the return of military forces to their permanent duty stations and the establishment of dialogue between Moscow and the authorities in Ukraine. (Reporting by Michael Shields and Georgina Prodhan; Editing by Hugh Lawson)