A Yemeni government official, who spoke in return for anonymity because he was not authorized to brief reporters, said Tuesday that the details of Mr. Saleh’s proposal were “still in the works.” The opposition parties’ plan, proposed earlier this month, urged Mr. Saleh to complete arrangements by the end of the year for his early departure. But the parties have since backed away from the offer, joining with street demonstrators calling for Mr. Saleh to quit immediately.

The United States again expressed concern on Tuesday that a power vacuum in Yemen could provide an opening for terrorist groups, including Al Qaeda’s local affiliate, Al Qaeda in the Arabian Peninsula, which since 2009 has mounted multiple terrorist plots against the United States.

“We are obviously concerned about the instability in Yemen,” Defense Secretary Robert M. Gates said Tuesday while traveling in Russia. “We consider Al Qaeda in the Arabian Peninsula, which is largely located in Yemen, to perhaps be the most dangerous of all of the franchises of Al Qaeda right now.”

Obama administration officials acknowledged that they were watching events unfold in Sana with relatively little ability to influence them. They have limited their official statements to condemnations of violence and calls for a peaceful move to more democratic rule.

Mr. Saleh’s indication on Tuesday that he might be willing to step down early came after four top military commanders, including General Ahmar, joined a wave of defections by diplomats and other officials. Previously, Mr. Saleh had offered to leave by 2013, when his current term ends.