(Adds U.N. background, Biden calling for U.S. force)

By Sue Pleming

WASHINGTON, April 11 (Reuters) - The United States has delayed for several weeks imposing new sanctions against Sudan over its handling of Darfur to give the United Nations more time to negotiate with Khartoum, the U.S. special envoy to Sudan said on Wednesday.

Special envoy Andrew Natsios told lawmakers U.N. Secretary-General Ban Ki-moon had asked the United States at the end of last month to wait for two to four weeks to enable him to negotiate a U.N.-African Union peacekeeping force for Darfur, which Sudan has so far refused.

African Union officials plan to come to the United Nations on Monday to complete planning for the AU-U.N. hybrid force.

"As a courtesy to the (U.N.) secretary general, we delayed," Natsios told the Senate Foreign Relations Committee. "I am going to give him the four weeks."

Lawmakers criticized the move, saying lives were at risk in Darfur, particularly in camps for the more than 2.5 million displaced by the conflict since 2003.

"If I was sitting in those camps, I could not stand the counsels of patience and delay," said Sen. Robert Menendez of New Jersey, a Democrat.

"People are bleeding to death now," said Sen. Joe Biden, a Democrat from Delaware who heads the committee. "It is genocide, we should act now."

Biden said the United States should use military force against Sudan, an option the Bush administration says is not viable at this point.

The United States was set last month to impose stricter sanctions to pressure Sudanese President Omar Hassan al-Bashir to agree to allow the peacekeeping force into Darfur.

Among new U.S. sanctions is the addition of 29 Sudanese companies to a current U.S. sanctions list of about 130 firms.

Washington also plans to further limit dollar transactions from Sudanese companies and to slap travel and banking bans on three individuals, including an "obstructionist" rebel leader.

Another strategy was to more aggressively enforce sanctions on Sudan, using similar tools as those employed to put pressure on Iran and North Korea.

"We believe it will have a substantial effect (on Sudan's economy)," Natsios said of planned new sanctions.

Britain, which has been preparing a new resolution in the U.N. Security Council against Sudan, was also asked to wait until at least next week, U.N. diplomats said.

The proposed U.N. sanctions are expected to include an arms embargo, financial or travel bans on targeted individuals and institutions, and some monitoring of military overflights in Darfur, diplomats said.

Natsios hoped European nations would also take action, including restrictions on euro transactions in Sudan.

Biden asked Natsios why a no-fly zone had not been imposed over Darfur, where more than 200,000 people have been killed since 2003.

Natsios declined to discuss military options, except to repeat the Bush administration's view that "all military options are on the table."

Britain has also pushed for a no-fly zone in Darfur but U.S. defense officials say this is not an option being actively explored as such a measure would be hard to implement in an area the size of Texas.

The U.S. envoy said the situation on the ground in Darfur had become "increasingly chaotic," and the "dangerously regionalized" war has destabilized Sudan's neighbor, Chad.

U.S. Deputy Secretary of State John Negroponte is set to leave on Wednesday for Sudan, Chad and Libya in a bid to ease the crisis in Darfur. He is due back in Washington on May 19 and any sanctions are unlikely until the end of his visit.

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