A UN spokesman rejected the call by Senator Sam Brownback of Kansas and Representative Frank Wolf of Virginia, saying Annan was not in a position to force action.

The two lawmakers also criticized a UN report released Monday for failing to recommend any action to stop the killing, saying the report had been sabotaged by internal UN politics.

"We cannot wait any longer for credible action on Darfur," Brownback said. "The time is now for the secretary general, Kofi Annan, to lead or leave."

The two Republican lawmakers said stopping the killing in the western region of Sudan would be a test of whether the United Nations remains relevant in the 21st century. Some 70,000 people are estimated to have died, many from hunger and disease, while 1.5 million others have been displaced, many into squalid and dangerous camps, in two years of conflict.

"The United Nations has failed over and over to prevent genocide," said Wolf, who heads the House of Representatives subcommittee that approves US payments to the United Nations.

"If in the year 2005 the Security Council cannot deal with genocide ... I believe it's fair to ask what purpose is the United Nations serving in the 21st century."

Brownback said Security Council action "needs to happen within the next couple of weeks."

The two lawmakers said they want the council to: impose sanctions on the government in Khartoum, including a worldwide arms embargo; bar investments in Sudan; and increase sharply the number of African Union peacekeepers in Darfur.