BAGHDAD, April 3 (Reuters) - The U.S. ambassador to Baghdad said on Thursday he was caught off guard when a big battle broke out between Iraqi forces and militias in Basra last week, saying U.S. troops were only able to give support after fighting began.

Ryan Crocker, just days away from delivering key testimony to the U.S. Congress on Iraq, sought to paint a positive picture of Prime Minister Nuri al-Maliki's crackdown on Shi'ite militias that many analysts say highlighted the weakness of Iraq's army.

Iraq's defence minister has said his forces were not ready for such fierce resistance from the Mehdi Army militia of populist Shi'ite cleric Moqtada al-Sadr in the southern city.

The Interior Ministry has said 210 people were killed in Basra during a week of fighting. The operation sparked clashes in other towns across southern Iraq and in Baghdad.

At a briefing with foreign journalists before leaving for Washington on Friday, Crocker was asked about how much he knew of the operation in advance.

"I had the understanding this was going to be an effort to kind of get down, show they were serious with additional forces, put the squeeze on, develop a full picture of conditions and then act accordingly," Crocker said.

"I was not expecting frankly a major battle from day one. But then again it's not clear to me that they'd decided that's what they were going to do. The enemy has a vote in combat."

The scale of the fighting -- U.S. and British forces had to launch air and artillery strikes to support Iraqi troops -- comes at an awkward time for Crocker and U.S. commander General David Petraeus who will also testify to Congress on April 8-9.

Both men are likely to face questions from opposition Democrats about the ability of Iraq's forces to function with less U.S. military support as American troop levels decline.

Petraeus is expected to recommend a pause in withdrawals when an initial 20,000 troops depart Iraq by July. That will leave around U.S. 140,000 soldiers in the country.

Asked at a separate news conference about the apparent lack of notice given to the Americans, Maliki said: "We told them frankly these are purely Iraqi operations and this is the way that other operations will be in the future."

"BOATLOAD OF PROBLEMS"

Crocker avoided being drawn on how last week's fighting might affect his testimony next week.

He rejected suggestions Maliki went into Basra hastily, saying he was impressed the prime minister acted "decisively".

"Were there a lot of problems? There were a boatload of problems and there is still a long way to go," Crocker said when asked about the capability of Iraq's security forces in light of the operation.

"It's true that as far as support we were able to give, we kind of had to put that together after the battle was joined."

But Crocker said Iraq's forces were able to plan and execute the operation and make adjustments when they ran into trouble.

Sadr on Sunday pulled back from all-out confrontation, ordering his militia to stop fighting but to ignore Maliki's order to surrender heavy weapons. A fragile calm has since held.

The young cleric backed Maliki's rise to power in 2006 but split with the prime minister a year ago, partly over Maliki's refusal to set a timetable for the withdrawal of U.S. forces.

Crocker said the Basra violence did not "erase the significant progress" in security over recent months, but added: "Gains are fragile. This episode demonstrates it."