Rolf Ekeus, the Swedish diplomat who oversaw the disarmament of Iraq from 1991 to 1997, was nominated today by United Nations Secretary General Kofi Annan to head up a new agency responsible for eliminating Baghdad's remaining long-range missiles and chemical and biological weapons programs.

But Moscow immediately challenged the nomination, saying the Swedish ambassador to Washington is "unacceptable" because of his past association with the U.N. Special Commission (UNSCOM), which has been credited with destroying more Iraqi weapons than the Gulf War coalition.

Russia's move is likely to set the stage for a battle in the Security Council with the United States, which strongly approves of Ekeus.

A senior U.S. official said Russia's decision to oppose Annan's choice represented an "unprecedented" gesture of defiance. While he acknowledged that Moscow has the power to stall or block the appointment, he said Washington would mount a sustained political battle to make the appointment stick, or at least to ensure that the post goes to a suitable candidate to head the U.N. Monitoring, Verification and Inspection Commission, created last month to succeed UNSCOM.

Annan's choice of a nominee without the council's backing comes after a month of frustrating efforts to obtain universal support for a single candidate. In the past month, key council members have failed to agree on some 25 candidates.

Ekeus's appointment was opposed by three council members with veto power--Russia, France and China--during private consultations with Annan last week, on the grounds that he was unacceptable to Baghdad. French and Chinese officials in New York said today that they were awaiting instructions from their capitals before reacting to the announcement.

But they made it clear that Ekeus didn't have their support. "We are not enthusiastic," one diplomat said.

Richard C. Holbrooke, U.S. ambassador to the United Nations and this month's council president, distributed a draft letter to his counterparts today approving Ekeus's appointment and asking him to begin work as soon as possible. But in a written reply to Holbrooke, Sergei Lavrov, Russian ambassador to the United Nations, said, "The Russian federation cannot agree with the proposal in the absence of consensus." Holbrooke scheduled a Security Council meeting tomorrow afternoon to debate the council's next step

UNSCOM, which was established after the Gulf War to account for and destroy Iraq's most lethal weapons, used highly controversial intelligence-gathering methods to thwart Baghdad's efforts to conceal prohibited weapons.

Iraq has refused to allow inspectors to return since December 1998, when they were evacuated on the eve of a U.S. and British bombing campaign, and has accused them of spying for the United States.