Children climb a destroyed Iraqi Army armored vehicle Saturday in the Shiite enclave of Sadr City, Baghdad. The armored vehicle was destroyed in two weeklong clashes in Baghdad's Sadr City.

Baghdad ? A roadside bomb killed an American soldier in Baghdad on Saturday, capping the bloodiest week for U.S. troops in Iraq this year. Clashes persisted in Shiite areas, even as the biggest Shiite militia sought to rein in its fighters.

At least 13 Shiite militants were killed in the latest clashes in Baghdad’s militia stronghold of Sadr City, the U.S. military said. Iraqi police said seven civilians also died in fighting, which erupted Friday night and tapered off Saturday.

The U.S. military said the American soldier was killed in a blast Saturday morning in northwestern Baghdad but did not say whether Shiite militiamen were responsible.

The death raised to at least 19 the number of American troopers killed in Iraq since last Sunday.

American casualties have risen with an outbreak of fighting in Baghdad between U.S. and Iraqi forces and the largest Shiite militia – the Mahdi Army of anti-American cleric Muqtada al-Sadr.

Al-Sadr, who is believed to be in Iran, repeated on Saturday his demand for American soldiers to leave the country and urged his fighters not to target fellow Iraqis “unless they are helping the (U.S.) occupation.”

Al-Sadr also blamed the Americans and their Iraqi allies for the assassination Friday of one of his top aides, Riyadh al-Nouri, director of his office in the Shiite holy city of Najaf.

Gunmen ambushed al-Nouri as he was returning home from Friday prayers, and al-Sadr followers shouted anti-American slogans at his funeral in Najaf.

Despite the strident rhetoric, however, there were signs that al-Sadr was trying to calm his militia to avoid all-out war with the Americans. Al-Sadr is also under pressure from Prime Minister Nouri al-Maliki, also a Shiite, to disband the Mahdi Army or face a ban from politics.

Sadrist officials told The Associated Press they had received orders from their headquarters in Najaf to avoid confrontations with Iraqi and U.S. forces unless the Americans try to move deep into Sadr City, which has been under siege for two weeks.

Elsewhere, Iraqi soldiers acting on tips from detained Shiite militiamen found 14 bodies that had been buried in a field south of Baghdad, officials said Saturday. It was the second discovery this week of mass graves in the area, raising to 45 the number of bodies located there.