Once the drawdown of troops ends in July, Petraeus recommended a 45-day period "of consolidation and evaluation ... (then) a process of assessment to examine the conditions on the ground" to help determine when reductions could resume.

"This approach does not allow establishment of a set withdrawal timetable. However, it does provide the flexibility those of us on the ground need to preserve the still-fragile security gains our troopers have fought so hard and sacrificed so much to achieve," Petraeus told the Senate Armed Services Committee.

When committee Chairman Sen. Carl Levin, D-Mich., asked for a definitive time line, Petraeus said reductions would resume "when the conditions" are right.

Gains have been made in Iraq since his last appearance before Congress last fall, but "challenges remain," Petraeus said.

Ethno-sectarian violence, he said, "is a cancer that will continue to spread if left unchecked."

"The progress made since last spring is fragile and reversible," he said. "Still, security in Iraq is better than it was seven months ago" and better than before the surge began.