Najaf, Iraq ? The sound of heavy gunbattles resonated today throughout the holy city of Najaf, as U.S. forces battled militia loyal to radical Shiite cleric Muqtada al-Sadr.

It wasn’t clear whether the stepped-up fighting signaled the beginning of a major offensive against al-Sadr’s Mahdi Army, but the U.S. military and Iraqi forces had earlier said they were preparing for a fullscale assault to crush the weeklong uprising.

The planned offensive could inflame Iraq’s Shiite majority — including those who do not support the uprising — if it targets the revered Imam Ali shrine, where many of the insurgents have taken refuge.

Any assault on the shrine likely would include or be led by the Iraqi forces — many of whom have only minimal training — in an effort to lessen the anger. The U.S. military said Wednesday it was holding joint exercises with Iraqi national guardsmen in preparation for the planned assault.

“Iraqi and U.S. forces are making final preparations as we get ready to finish this fight that the Muqtada militia started,” said Col. Anthony Haslam, commanding officer of the 11th Marine Expeditionary Unit.

As clashes in Najaf raged for a seventh day, U.S. troops said they were impressed with militants’ tenacity.

“I think they got a reproduction facility down there. I think they’re cloning,” Capt. Patrick McFall said. As he spoke, a mortar exploded nearby, sending up plumes of black smoke.

In response to the announcement, al-Sadr loyalists in the southern city of Basra threatened to blow up the oil pipelines and port infrastructure there if coalition forces launched a major attack in Najaf. A similar threat Monday caused oil officials to briefly stop pumping from the southern oil wells.

The firebrand cleric leading the insurgents has exhorted his followers to fight on even if he is killed.

The U.S. military has estimated that hundreds of insurgents have been killed in the Najaf fighting, but the militants dispute the figure. Five U.S. troops have been killed, along with about 20 Iraqi officers.

Al-Sadr’s forces continued to fight coalition forces and Iraqi authorities in other Shiite communities across Iraq.

Clashes between insurgents and British forces in the southern city of Amarah killed 20 people and wounded 50, according to Interior Ministry spokesman Adnan Abdul Rahman. The British reported two minor casualties among their own troops.

In Kut, 100 miles southeast of Baghdad, Iraqi police fought off attacks from al-Sadr’s Mahdi Army at the central police station and other government offices. The fighting killed four people and wounded 20, said Dr. Falah al-Bermany.