* China, Russia said to oppose formal council discussion

* U.N. says rebels hold civilians as human shields

UNITED NATIONS, March 26 (Reuters) - The U.N. Security Council will discuss the humanitarian situation in Sri Lanka on Thursday as fears grow about the fate of tens of thousands of civilians trapped by fighting between government forces and rebels, diplomats said.

U.N. humanitarian affairs chief John Holmes is set to brief council members at an informal meeting that is not expected to lead to any action, diplomats on the 15-nation council told reporters.

Diplomats from the United States, Britain, France, Austria and other council member states have said they would be open to such a briefing.

But U.N. diplomats say China and Russia were among those who oppose formal discussion of the issue, saying the fighting between the separatist Liberation Tigers of Tamil Eelam (LTTE) and government forces represents no threat to international peace and security and was therefore no business of the council.

Sri Lanka's military said on Thursday it was close to trapping the Tigers in a no-fire zone, along with thousands of civilians at grave risk in the 25-year war that authorities say is in its final act. [ID:nCOL84177]

The United Nations says the Tigers have forcibly kept people there as human shields or conscripts, and has warned the government against shelling the safe zone. It says 2,800 civilians have been killed since Jan. 20.

The government says it is not firing into the no-fire zone and that the U.N. numbers are unsubstantiated. The Tigers say people are choosing to stay with them.

Human Rights Watch (www.hrw.org) has accused the government of indiscriminately shelling the no-fire zone where the civilians are. It also said the Tigers were forcing most civilians to stay. (Reporting by Louis Charbonneau; Editing by Xavier Briand)

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