By Susan Cornwell

WASHINGTON, July 19 (Reuters) - The U.S. ambassador to Iraq warned skeptical senators on Thursday against an unconditional withdrawal of American troops but was told, in the words of one influential Democrat, "We ain't staying."

Speaking the day after Democrats failed in their latest attempt to get enough Republicans to advance a troop withdrawal plan in the Senate, Ryan Crocker warned that pulling U.S. forces out of Iraq without conditions could provide "more running room for Iran" to expand its influence in the region.

It could also create a "comfortable operating environment" for al Qaeda and cause a sharp spike in sectarian violence in Baghdad that President George W. Bush's addition of thousands more troops this year was meant to quell, he said.

Crocker, who has been in Baghdad less than four months, spoke to the Senate Foreign Relations Committee via video conference from the Iraqi capital. To the amusement of anti-war protesters, listening along with senators in a packed committee room, the link broke down for several minutes at one point.

Sen. George Voinovich of Ohio, a Republican who has distanced himself from Bush's Iraq policy, bluntly told Crocker: "We have to disengage. It's inevitable."

Sen. Joseph Biden, a 2008 Democratic presidential hopeful from Delaware who chairs the committee, chimed in: "Listen to the Republicans. We ain't staying. We're not staying. We're not staying. Not much time."

Crocker spoke earlier on Thursday to several dozen members of the House of Representatives in a closed question-and-answer session at the Pentagon.

But he faced an especially tough audience in the public Senate hearing, where a majority of the committee has expressed doubts or outright opposition to Bush's strategy of adding 30,000 more troops to Iraq this year. There are about 160,000 U.S. troops in Iraq now.

Two of Bush's fellow Republicans on the panel, Voinovich and Indiana's Richard Lugar, have called on Bush to produce a strategy to start drawing down troops.

But Crocker said that if there were administration efforts to develop a "plan B" for a pullout of U.S. combat forces, he was not aware of them.

Another Republican, Nebraska's Chuck Hagel, has voted since early this year with Democrats who want to wind down the war.

Some senators were irritated by Crocker's assertion that they shouldn't depend too much on the benchmarks for Iraqi political progress that they have written into law to help determine how long the United States should stay in Iraq.

But Crocker told them he did not want to sugar-coat the situation. "If there is one word I could use to sum up the atmosphere in Iraq ... that word would be fear," Crocker said.

He said there had been some progress in Iraq, with violence down notably in Baghdad since the full complement of additional troops arrived last month.

Although U.S. troops could not stay forever, they needed to be able to stay long enough for Iraqi forces to be able to protect the population themselves, Crocker told the panel.

Lawmakers may have to wait longer than they thought for a full assessment from the administration of whether Bush's strategy is working, a top U.S. commander said on Thursday.

A September report to be produced by Crocker and Iraq commander Gen. David Petraeus will show whether the plan is progressing, but a fuller assessment will take until November, Lt. Gen. Ray Odierno, the deputy commander in Iraq, said.

(Additional reporting by David Morgan and Richard Cowan)

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