WASHINGTON -- As rebels rack up important victories that could hasten the fall of Syrian President Bashar Assad, U.S. officials are still struggling to identify a credible opposition authority to keep fragile Syria from civil war once the leader is gone.

The main opposition groups Washington supports lack cohesion, credibility and, most importantly, command over the armed rebels who on Friday said they were sending reinforcements to Damascus for battles that could determine whether the four-decade Assad family dynasty survives.

The U.N. Security Council voted Friday on a 30-day extension of special envoy Kofi Annan’s troubled monitoring mission, although prospects for a last-ditch diplomatic resolution seemed low given the escalating violence on the ground.

The rebel forces’ most significant attack so far – a bombing Wednesday that killed three top defense officials – claimed a fourth high-profile target on Friday, when Syrian state television announced that national security chief Hisham Ikhtiar died of wounds he suffered in the blast.

Joseph Holliday, an analyst of the Syrian insurgency for the Washington-based Institute for the Study of War, said U.S. officials don’t believe “meaningful, political negotiations between the regime and the opposition” would ever occur. That means Assad’s removal almost certainly will come by force, and with no obvious government-in-waiting, there are scant details on what the day after would look like.

Since President Barack Obama called for Assad’s ouster nearly a year ago, the United States and its allies adamantly have resisted military intervention, U.N.-led diplomatic efforts have collapsed and the regime has successfully kept most foreign media from reporting a clear picture of the muddiest of the Arab Spring uprisings.

“Who are we even talking to?” Holliday said, referring to the U.S. government’s risk of pinning hopes on shadowy opposition figures who might be unable to deliver on the ground.

A State Department official, speaking on condition of anonymity under diplomatic protocol, said American diplomats were frustrated that the opposition forces still haven’t managed to coordinate better with the rebel fighters or work out the internal differences that have led to a mosaic of competing forces – homegrown vs. exiles, secular vs. Islamist, armed vs. pacifist.

At the same time, the official added, “the U.S. hasn’t wanted to knight someone” because it was important for the process to be seen as an all-Syrian undertaking.

“‘He must go,’ then begs the question, ‘Who comes next?’” the official said. “And if we don’t have an answer to that, it’s hard to go further.”

Syrian academics and technocrats – almost all of them exiles – who were tasked with creating a shadow government don’t appear to have real support on the ground in Syria, in Washington or at the United Nations, according to analysts and published remarks by officials.

In a war game exercise last month, senior analysts in Washington who specializing in the Middle East played out scenarios for getting rid of Assad. In a grim report stemming from the exercise, the most pessimistic words were reserved for the prospects of a transitional administration.