WASHINGTON - The top U.S. commander in Iraq, Gen. David H. Petraeus, urged the Iraqi government to seek conciliation with Shiite leader Muqtada al-Sadr, saying the cleric headed a "legitimate political movement" that could play a constructive role.

Petraeus expressed worry that the recent clashes between Sadr's militia and government forces in Baghdad and Basra could undermine an eight-month-old truce between Sadr's Mahdi Army militia and U.S. forces that has contributed to a decline in violence.

"We are concerned that the cease-fire could fray," Petraeus said. "There has to be a very, very sensitive approach as this goes forward, to make sure that folks don't feel like they're backed into a corner from which there's no alternative."

Petraeus expressed his concerns even as Prime Minister Nouri al-Maliki called a meeting in Baghdad of the main political blocs, both Shiite and Sunni, to gain support for his crackdown on Shiite militias, but pointedly left out the Sadrists, the Associated Press reported.

The absence of the Sadrists signaled that Maliki was making good on a threat to try to isolate the movement politically if its Mahdi Army militia is not disbanded.

Maliki, a Shiite, has found himself on the defensive after Iraqi government forces were surprised by the fierce resistance by Shiite militias to the offensive he ordered against them March 25 in Basra and elsewhere.

Sunni politician Adnan al-Dulaimi, who leads the largest Sunni bloc in parliament, emerged from the meeting to say the operation against Shiite militias was "a courageous step."

The meeting also was attended by Shiite lawmakers Hadi al-Amiri and Khalid al-Attiyah, the deputy parliamentary speaker.

But the meeting drew warnings from Sadrist lawmakers that the government's effort to isolate them could backfire.

"The Iraqi prime minister is waging a political war," one Sadrist lawmaker, Falah Shanshal, said. "But he is committing a big mistake because the Sadr movement enjoys the support of a large portion of the Iraqi public."

While fighting between Shiite militants and U.S.-Iraqi forces has eased somewhat after days of fierce clashes in Baghdad's Sadr City district, it has taken its toll on all sides.

The U.S. military announced that an American soldier was killed by a roadside bomb Wednesday in central Baghdad, raising to 18 the number of Americans killed in Iraq so far this month.

A marked reduction in casualty rates began around September 2007, and daily averages continued to decline through the rest of that year. However, since reaching a low in January of 20 Iraqis killed per day, casualty levels have once again started to rise, with 26 killed per day in February and 41 per day in March.

In the first nine days of April, at least 261 Iraqi civilians and security personnel were killed or found dead across Iraq, an average of 29 per day, an Associated Press tally showed.