WASHINGTON, April 8 (UPI) -- U.S. Army Gen. David H. Petraeus called progress in Iraq "fragile and reversible" during an appearance Tuesday before the Senate Armed Services Committee.

Petraeus made a widely anticipated recommendation that U.S. forces be pulled back to a pre-surge level this summer but said that cut should be followed by a 45-day period of evaluation.

He declined to estimate whether any further troop reductions would be made following the summer "pause" or what level of U.S. troops might remain in the country by the end of this year. Withdrawing troops too quickly could jeopardize the progress since the surge began last year, he said.

Petraeus, the top U.S. military officer in Iraq, appeared alongside Ryan C. Crocker, the top U.S. diplomat in Iraq, to update the Senate on the Iraqi government's military and political progress.

Petraeus said the security progress in Iraq has been significant but uneven, with improvements that include a decrease in overall violence and civilian deaths to the lowest level since 2005. He said many Iraqis turned against the violent methodology of groups such as al-Qaida in Iraq.