Nigeria ? U.S. Deputy Secretary of State Robert Zoellick rushed to Nigeria on Monday, hoping to break a crucial impasse in peace talks over Sudan’s Darfur region.

Zoellick’s unexpected trip came a day after thousands rallied in Washington urging the Bush administration to do more to end the war in Darfur, and amid fading hopes that rebels would sign a peace plan backed by the Sudanese government before a deadline tonight.

The rebels held fast Monday to their demands for greater political representation and security guarantees.

At peace talks in Abuja, Nigeria, the leader of Sudan’s delegation, Vice President Ali Osman Taha, left the negotiations to return to Sudan. It wasn’t clear whether he would return, and some analysts suggested that his departure – despite overtures from the rebels that he stay and discuss their differences – meant that the Sudanese government thought it had won the diplomatic high ground and wouldn’t cede much to the rebels.