U.S. Ambassador Richard C. Holbrooke urged the United Nations today to prepare to send peacekeepers to the West African country of Sierra Leone before the end of the year. But U.S. officials said no American troops would participate.

Sierra Leone, a diamond-rich nation of 4.8 million people, has been the scene of horrific atrocities during an eight-year civil war. U.N. Secretary General Kofi Annan appealed to the Security Council last week to authorize a force of 6,000 peacekeepers to oversee the voluntary disarmament of more than 40,000 combatants under a peace agreement signed this year by the government and rebel forces.

U.S. officials said Holbrooke's endorsement of the mission was part of a broader push to help the United Nations play a more active role in resolving African conflicts from Sierra Leone to Congo and the border war between Ethiopia and Eritrea. But some U.N. diplomats remain skeptical about Washington's commitment to African problems.

"There is a very strong feeling among policy-makers that Rwanda, which is the most significant genocide since the Holocaust, was something that happened under our watch," said Chester Crocker, the former U.S. assistant secretary of state for African affairs. "We're saying 'yes' in East Timor, and we said 'yes' in the Balkans, but we have said 'no' repeatedly in Africa."

A Nigerian-led peacekeeping force of 12,000 troops, known as ECOMOG, is operating in Sierra Leone, but Nigerian President Olusegun Obasanjo has informed the United Nations that he will begin withdrawing 2,000 troops each month starting in October.

"We are anxious to begin disarmament and demobilization in Sierra Leone," Holbrooke said during a special Security Council debate on Africa. "We need the U.N. to . . . be ready to introduce a full peacekeeping operation in December, when the Nigerians plan to leave."