WASHINGTON (Reuters) - The top U.S. commander in Iraq told Congress on Tuesday he plans to stop U.S. troop withdrawals in July due to fragile security gains and heard appeals for quicker action to find a way to end the war.

Appearances by Gen. David Petraeus and the U.S. ambassador to Iraq, Ryan Crocker, drew U.S. presidential candidates eager to be heard on an issue that is among top concerns of war-weary American voters ahead of the November election.

Petraeus gave them a cautious assessment. “We haven’t turned any corners, we haven’t seen any lights at the end of the tunnel. The champagne bottle has been pushed to the back of the refrigerator. And the progress, while real, is fragile and is reversible,” he told the Senate Armed Services Committee.

While Republican candidate Sen. John McCain said current policy is succeeding, Democratic senators Barack Obama and Hillary Clinton called for faster troop withdrawals, a move opposed by the two top U.S. officials in Baghdad.

They made their assessment a year after thousands more U.S. troops were poured into Iraq and after a new outbreak of violence in recent weeks, including the deaths of 11 American service personnel in the past 48 hours.

Petraeus told two Senate committees there has been an improvement in security in parts of Iraq but that the gains are uneven.

To avoid jeopardizing the gains of the past year, he said he had recommended a 45-day halt in July to a series of troop withdrawals. After that pause, he would assess conditions on the ground to determine whether security is sufficient to bring more troops home.

WITHDRAWAL TIMETABLE

He resisted any sort of timetable to bring the troops home, a position advocated by Democrats.

The United States now has 160,000 troops in Iraq. Under plans announced last year, the Pentagon is pulling five combat brigades -- or about 20,000 troops -- out by mid-July, bringing the force level down to what it was before the troop increase.

The end result is that more than 100,000 U.S. troops could still be in Iraq until Bush leaves office in January 2009, leaving the U.S. presence to the next president to handle.

Illinois Sen. Obama told Petraeus and Crocker at a Senate Foreign Relations Committee hearing that Washington needs to increase pressure on Iraqis to resolve their differences.

“Increased pressure in a measured way, in my mind, and this is where we disagree, includes a timetable for withdrawal. Nobody’s asking for a precipitous withdrawal, but I do think that it has to be a measured but increased pressure, and a diplomatic surge that includes Iran,” Obama said.

New York Sen. Clinton, who is battling Obama for the Democratic Party presidential nomination, told Petraeus at an Armed Services Committee hearing that the United States should begin withdrawing its troops from Iraq to focus on problems elsewhere.

“I think it’s time to begin an orderly process of withdrawing our troops, start rebuilding our military, and focusing on the challenges posed by Afghanistan, the global terrorist groups and other problems that confront Americans,” she said.

But Arizona Republican Sen. McCain said he saw a genuine prospect of success in Iraq and warned that defeat could require U.S. troops to return in a broader war.

“We’re no longer staring into the abyss of defeat and we can now look ahead to the genuine process of success,” said McCain, a strong supporter of the U.S. presence who has clinched his party’s presidential nomination.

Slideshow ( 34 images )

POLITICAL END GAME

The unflappable Petraeus, dressed in a medal-bedecked uniform, and the urbane Crocker faced tough questioning and critical comments all day long. Even Republicans sounded frustrated by the pace of progress in Iraq.

“I think what people want a sense of is what the end is going to look like,” said Tennessee Republican Sen. Bob Corker.

Ohio Republican Sen. George Voinovich said the war is costing many billions and “in effect we’re kind of bankrupting this country.”

Slideshow ( 34 images )

He said Syria, Saudi Arabia and Egypt need to be told, “Hey guys, we’re on our way out. We have to leave here because of our own financial situation.”

Crocker said he shared the frustration.

“If you decide ... that we just don’t want to do this anymore, then we certainly owe ourselves a very serious discussion of ‘then what?’ What are the consequences? Because my experiences in the Middle East... frankly are that things can get really, really bad indeed,” he said.

Crocker said major changes in U.S. policy could allow al Qaeda to gain strength in Iraq and permit Iran to increase its influence. “I remain convinced that a major departure from our current engagement would bring failure,” he said.

Petraeus’ plan to stop troop withdrawals drew a rebuke from the Armed Services Committee chairman, Michigan Democratic Sen. Carl Levin. He called it “an open-ended pause” that would represent “the next page in a war plan with no exit strategy.”

Levin demanded to know how many U.S. troops would be in Iraq at the end of 2008.

“Sir, I can’t give you an estimate,” said Petraeus.

Protesters several times interrupted the proceedings, providing an edgy atmosphere inside a Capitol Hill hearing room packed with news media and onlookers.

“Bring them home!” shouted one demonstrator to scattered applause who was hustled out as Petraeus tried to speak.