NAJAF, Iraq - Insurgents fired mortar shells at U.S. forces moving into Najaf's vast cemetery and American jets roared overhead Wednesday, as the radical cleric leading the fight against coalition forces urged his followers to battle on even if he is killed.

Fighting in Najaf entered its seventh day, with Iraqi police manning checkpoints that cut the holy city in two. No U.S. casualties were reported in the fighting, which took place near the Imam Ali shrine, one of the holiest in Shia Islam.

On Tuesday, U.S. helicopter gunships pummeled a multistoried building 400 yards from the gold-domed Imam Ali Shrine with rockets, missiles and 30mm cannons - one of the closest strikes yet. The military said about 20 people were killed inside the building.

But by Wednesday more militants had entered the scorched hotel to resume firing at troops.

"We keep pushing south and they just keep coming," said Capt. Patrick McFall, from the 1st Cavalry Division.

Ibrahim al-Jaafari, Iraq's new interim vice president, called on the U.S. troops to withdraw from Najaf.

"Only Iraqi forces should stay in Najaf, these forces should be responsible for security and should save Najaf from this phenomenon of killing," al-Jaafari told Arab TV network Al-Jazeera from London Wednesday.

Coalition forces said they were operating in the city at the request of the government.

Sporadic explosions could be heard elsewhere in the city. U.S. Marine Maj. David Holahan said militants with radical Shiite cleric Muqtada al-Sadr's Mahdi Army attacked three police stations Tuesday, two with small arms fire, one with eight mortar rounds.

"We've pretty much just been patrolling and flying helicopters all over the place, and when we see something bad, we blow it up," said Holahan, executive officer of the 1st Battalion, 4th Marines Regiment.

Elsewhere, a roadside bomb exploded in a market north of Baghdad today, killing at least six Iraqis and wounding nine others, a hospital official said. The explosion shook the market in Khan Ban Sad, about six miles south of Baqouba, northeast of Baghdad.

"The place was crowded, but there were neither police or American patrols during the time of the explosion, and we are investigating this issue," said Baqouba police Col. Adnan Hussein.

Al-Sadr's fighters have been battling coalition forces for days in a number of Shiite strongholds in Baghdad and southern Iraq.

"I hope that you keep fighting even if you see me detained or martyred," al-Sadr said in a statement today. "I thank the fighters all over Iraq for what they have done to set back injustice."

To control movement in Najaf, Iraqi police and national guards blocked roads that connect the city's northern and southern parts Wednesday.

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