Najaf, Iraq ? U.S. forces in Najaf came under their most intense attack yet by Shiite militiamen in a clash Monday that may have killed up to 20 Iraqis. But the military still held back from going after the militia’s leader, a radical cleric, to avoid angering Iraq’s Shiite majority.

U.S. troops in Najaf, south of the capital, clashed for hours with Shiite militiamen who barraged the Americans’ base with mortars overnight, then opened fire in the afternoon from several directions. Tank and machine-gun fire demolished a building that troops said was the source of shooting, raising a pillar of smoke. Apache attack helicopters circled but did not fire.

Before dawn Monday, militiamen shelled the troops with about 20 mortars, hitting in and around the base where U.S. troops replaced Spanish forces a week ago. There were no casualties.

The U.S. military is deployed at the base and outside Najaf to crack down on radical Shiite cleric Muqtada al-Sadr and his militia. But the troops have held back because the military fears angering Shiite Muslims, whose holiest site in Iraq — the Imam Ali Shrine — is about three miles from the U.S. base.

Al-Sadr’s forces have stepped up attacks recently — apparently either to pressure U.S. officials to negotiate or goad troops into retaliating. On Saturday, al-Sadr’s al-Mahdi Army militia attacked a military supply convoy outside the southern city of Amarah, killing two American soldiers.

The U.S. military has vowed to capture or kill the cleric, whose militia launched an uprising in early April across the south. Al-Sadr, who is accused of involvement in the killing of a rival cleric, is in his office near the Imam Ali Shrine, making any move to capture him risky.

Al-Sadr has sought negotiations through tribal mediators in Najaf but wants a resolution that would avoid his arrest. U.S. officials insist he be brought to justice.

Also Monday, a senior U.S. military official said seven U.S. soldiers have been reprimanded in the alleged abuse of Iraqi captives at Abu Ghraib prison. Brig. Gen. Janis Karpinski, commander of the prison, called the abuse “despicable” and said she did not know about it at the time.

A U.S. soldier from the 2nd Armed Cavalry Regiment shoots a grenade launcher at insurgents in Najaf, Iraq. The U.S. soldiers, based in two camps in the city of Najaf, fired back Monday after insurgents attacked U.S. camps with mortars, rocket-propelled grenades and other small arms.