In a statement, President Bush says he is taking the action, because the conflict in Darfur threatens the national security and foreign policy of the United States.

The asset freeze is being imposed on four Sudanese identified by the U.N. Security Council as being involved in organizing and carrying out atrocities in Darfur.

The president's order comes days before rallies are planned in Washington and throughout the United States to protest the three-year war in Darfur.

Celebrities such as Academy Award winning actor George Clooney are scheduled to speak at the rally.

Clooney, who just returned from a trip to the Darfur region, told reporters in Washington the world's attention needs to be focused on what he called the "first genocide of the 21st century."

"What we cannot do is turn our heads and look away and hope that this will somehow disappear, because if we do they will disappear and an entire generation of people will be gone," said Mr. Clooney. "Then only history will be left to judge us like Rwanda, the Balkans and Cambodia."

Joining Clooney was Senator Sam Brownback, who has sponsored a number of bills and resolutions in the U.S. Congress regarding the conflict in Darfur.

Brownback supports a plan to send up to 20,000 United Nations peacekeepers to Darfur to replace the 7,000 African Union troops stationed there now.

"We have got to get protection into this region, into Sudan and Chad," said Mr. Brownback. "We have got African Union troops, they are not enough, they don't have enough mobility, they don't have a broad enough mandate. We need more troops, more mobility, a stronger mandate, in that region of Sudan and Chad."

The government in Khartoum hasvehementlyopposed any effort to send U.N. or NATO troops to Sudan, saying such a move would violate the country's sovereignty.

Sudan's government is accused of supporting local militias in Darfur that have massacred civilians and destroyed villages.

Actor George Clooney says international soldiers are needed to help the millions of people displaced by the fighting who are now living in squalid camps in Sudan and neighboring Chad.

"If it is simply that we can just go to these camps of two million people and put forces around them with guys with guns to keep people from killing people," added Mr. Clooney. "The issue ultimately comes down to security, period. There are innocent people who are in harm's way in the most brutal of fashion every single day and if not NATO or the U.N. then I am not quite sure what they are there for."

Mediators are pressuring Sudan's government and rebel groups to end the fighting, and both sides are considering a draft peace agreement designed to end the conflict.

After nearly two years of unsuccessful talks, African Union negotiators have set next Sunday as the deadline for an agreement to be reached.