WASHINGTON —  The Pentagon's ability to reduce troop levels in Iraq will hinge on how well the Iraqis handle violent outbreaks like the recent operations in Basra, and how many U.S. troops are needed to assist them, the Pentagon's top military officer said Wednesday.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, would not label the operation a win or defeat for the Iraqi troops. But he said it did not indicate a need for additional U.S. troops in Iraq, and would not likely change the immediate recommendations Gen. David Petraeus will deliver to Congress next week.

Mullen said officials are still assessing the mixed results of the Basra operation, in which Iraqi-led forces battled Shiite militias. The fighting has fallen off, and violence had dipped to pre-clash levels.

The importance of the Basra combat in the ongoing U.S. military evaluations was underscored, however, by the fact that Petraeus, the top U.S. commander in Iraq, delayed his travel to Washington for several days this week to assess the situation.

"It's very clear that if we had this kind of violence for a sustained period of time, those are the kinds of conditions on the ground that must go into (the) assessment of, 'Do I have enough troops to do this?"' Mullen told a Pentagon press conference. "This was a particularly violent week. ... And it is the kind of violence and lack of security that would certainly drive an assessment of what we would do after that."

Many of Petraeus' expected recommendations have been rolled out over the past two months, including plans for a pause in troop cuts after July when the last of the five additional brigades ordered to Iraq last year have come home.

It is expected that Petraeus will give Congress some estimate of how many U.S. troops he believes will be needed in Iraq through the end of the year, and how many more brigades could be withdrawn without sending in replacements.

There are now 158,000 U.S. troops in Iraq, including 18 combat brigades — down from a peak of 20 brigades for much of the past year. By the end of July, military leaders have said those numbers would fall to 140,000 troops, including 15 combat brigades.

While Petraeus won't lay out his proposals to Congress until early next week, and Bush has yet to publicly endorse them, Mullen made it clear there is no question that troop cuts will cease after July, as commanders determine the effects of the lower troop levels on security in Iraq.

Mullen would not, however, say how long the assessment period would last. Others have said it could be as much as two months.

While Mullen praised Iraq leaders for taking on the Basra assault, he acknowledged concerns that it could have been planned better, and that some Iraqi forces did not perform well.

"We've been looking forward to ... a time when the Iraqi security forces would, in fact, take the lead and be aggressive in terms of providing for their own security," said Mullen.

Iraqi Prime Minister Nouri al-Maliki returned to Baghdad Tuesday after personally overseeing the crackdown, which he had launched with the promise of "a decisive and final battle."

Iraqi security forces failed to crush the militias — largely the Mahdi Army followers of cleric Muqtada al-Sadr. And, the uneven results leave Maliki politically damaged and raise doubts about the progress U.S. troops are making in training the Iraqi forces.

Mullen declined to say how many U.S. troops were involved in the Basra operation, but said Americans provided air and logistical support, surveillance and some planning assistance, and there also were U.S. training teams with some Iraqi units.

The fighting eased after al-Sadr called his fighters off the streets Sunday under a cease-fire deal brokered by Iran. The violence had spread to Baghdad, including deadly attacks on the heavily fortified Green Zone.

Mullen took aim at Tehran's continued influence in Iraq, saying that U.S. troops are still finding Iranian-made weapons and Iranian-trained insurgents.

"They're providing weapons which are killing our people. They are training Iraqis to do the same thing. And that influence is really, really negative."

In related comments, Mullen said that the ongoing troop commitments in Iraq are making it impossible for the U.S. to meet requirements for as much as two additional combat brigades in Afghanistan.

Those two extra brigades, he said, are in addition to the need for 3,000 more trainers for the Afghan security forces. And, he said they would be in addition to any NATO commitments currently being considered at a high-level summit attended by Bush.

"There are force requirements there that we can't currently meet," Mullen said. "We've had significant impact there, but we don't have enough forces there to hold in what is a classic counterinsurgency."

He said that the priority right now is filling the shortfall in trainers.

Mullen also said that there are other places around the world where U.S. training teams or troops could be helpful, but those also are precluded by the combat needs of the Iraq war.