Iraq appears on track to establishing sustainable security — a key step toward withdrawing U.S. troops — the top U.S. military officer said Monday after visiting the newly quiet Sadr City section of the capital.

Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, told reporters that important elements of a solution to the Iraqi war — including reduced levels of sectarian violence, political reconciliation and stronger Iraqi forces — are coming into view more than five years after the U.S. invasion.

He repeatedly stressed, however, that the improvements are fragile and could still be reversed.

Mullen's assessment was notably upbeat and comes as the last of five Army brigades that were sent to Iraq in 2007 as reinforcements amid escalating sectarian conflict and rising death tolls is heading home.

"From all I see, the security conditions are holding, the level of violence is down; we're down to a level that we haven't seen in over four years," Mullen said on his fourth visit to Iraq since becoming Joint Chiefs chairman last October. "That, then, ties into decisions to be made later this year about the level of forces. So I hope we can continue the drawdown" after a late-summer pause, he added.

There are now about 145,000 U.S. troops in Iraq, down from a peak last year of nearly 170,000.

Pressed to say how much longer it might take to reach a conclusion about the permanence of the security gains, Mullen declined to be pinned down.

"I really need to spend more time with the commanders here to get their current assessment of where we are," he said. "I don't think there's going to be a clear milepost that says, `Hey, we're there."'

Mullen said he planned to meet later this week with Gen. David Petraeus, the top U.S. commander in Iraq, as well as Ryan Crocker, the top U.S. diplomat. Petraeus told Congress in May that he might be able to recommend further troop reductions this fall, after he makes a fresh assessment in late summer.

He flew by helicopter to Sadr City after arriving in the capital on an overnight flight from Washington. He visited U.S. troops at a coalition observation post and strolled through a market in Sadr City.

"We saw extraordinary progress there," he said. "A few months ago no one could go into Sadr City. I was able to walk openly down a street that until recently was extremely unsafe, and I'm encouraged by that."

More broadly, Mullen said progress in Iraq has been remarkable over the past six to 12 months.

"Should that continue for another six to nine to 12 months, certainly we would be in a position to make some decisions based on that. Whether, at that point in time, it would be sustainable or irreversible is something that I think we have to try to figure out."

He cautioned, without being specific, that "there are events which could change that" brighter outlook.