Rebel cleric Moqtada Sadr called on his supporters Thursday to rise up anew against U.S.-led security forces, his spokesman announced, after a fragile two-month truce in the holy city of Najaf ended with clashes that brought down a U.S. helicopter.

"This is a revolution against the occupation force until we get independence and democracy," the spokesman, Ahmed Shaybani, said in a telephone interview.

Sadr's militia, the Mahdi Army, said it had gained control of Najaf and three other southern cities -- Amarah, Nasiriyah and Basra -- where fighting has spread. Iraqi officials denied that the fighters had taken the cities. There was no independent confirmation.

Sadr's call for an uprising is his first significant test of Iraq's new interim government, which took office June 28, and signals the end to the uneasy peace that had settled over Iraq's long-oppressed Shiite Muslim majority in the southern part of the country.

It wasn't clear whether the call would result in a broad revolt. In April, Sadr rallied his supporters to strike against the U.S. occupation, leading to two months of clashes that left hundreds dead. Many here said they would listen closely to the messages Shiite clerics deliver during Friday prayer services for an indication of what might happen this time. But at the very least, the battles Thursday produced more anxiety in a country where the volatile security situation has already frayed nerves.

Sadr's followers also battled U.S. forces in the Sadr City district in Baghdad on Thursday, and gunfire could be heard at a nearby checkpoint.

[Separate attacks by Sadr's forces in Baghdad on Thursday wounded 15 American soldiers, the U.S. command said Friday, the Associated Press reported.]

The U.S. military and Iraqi police said the fighting in Najaf began when Mahdi Army fighters attacked a police station overnight. The military said Iraqi forces called for help to resist the attack.

"If they want it to be war, let it be," said Ghalib Hashim Jazaeri, the police chief in Najaf. "We have enough men and equipment to defeat them."

"We are inside the city," he said into his radio as gunfire and mortar rounds exploded. "We are chasing them. They left, escaped." The radio crackled.

Each side blamed the other for breaking a truce negotiated in June to end the two-month uprising.

"They broke the truce," Jazaeri said. "They want to occupy the city. We cannot let them do that. If they attack us, we will defend ourselves."

Mahdi Army fighters in the streets of Najaf shot off grenades and set up roadblocks with mortar tubes and tires. A voice over a loudspeaker coming from the shrine of Ali, one of the holiest sites in Shiite Islam, urged residents to take up arms to defend the city.

Shaybani denied that Sadr's followers had started the fight. He said the Iraqi police and National Guard and U.S. forces had broken the truce, which restricted U.S.-led forces from entering parts of the city, including areas near sacred sites.

Shaybani said the occupation forces surrounded the city about 2 p.m. Thursday. "We knew they wanted to invade it," he said. "We had and have to defend the holy city. We didn't want to violate the truce, and we are still committed to it. But they don't respect the word they gave. They want it to be war."

One U.S. soldier was killed and five were wounded in the fighting, the military said. Hussein Ali, a doctor at Najaf Hospital, said four Iraqi security officers and six civilians were killed in the battle, which wounded two Iraqi policemen and 18 civilians. The interim Iraqi government said eight Iraqi fighters were killed and 22 injured.

As a large plume of smoke rose from the city, a black U.S. helicopter tilted to one side and chugged slowly to the ground at an angle before it hit with a loud boom. Sadr's aides said the Mahdi Army shot down the aircraft, the Reuters news agency reported. The U.S. military said two crew members were wounded and had been evacuated.

In Basra, British soldiers engaged in a gun battle with members of the Mahdi Army after the soldiers were attacked by small arms fire. A British military spokeswoman said two militiamen were killed.

In Baghdad, Interior Minister Falah Naqib pledged to find and arrest Sadr.

"We will not negotiate," he said at a news conference. "We will fight these militias. We have power to stop these people, and we'll kick them out of the country."

Naqib said the decision for Iraqi forces to fight Sadr's militia came from the governor of Najaf province, not the U.S. military. On that point, Sadr's spokesman agreed.

"We were forced to do this after the governor started his stupid idea to invade the city," Shaybani said.

Also Thursday, insurgents attacked a police station in Mahawil, about 40 miles south of Baghdad. Two gunmen wearing police uniforms opened fire from a minivan as they approached guards outside the station. The gunmen jumped out of the van, and a third sped toward the station in another vehicle filled with explosives, killing five people and wounding 27, the Interior Ministry said. Assailants later attacked an Iraqi patrol in the city, killing one guardsman, the Associated Press reported.

In the northern city of Mosul, Iraqi security forces detained 13 suspected militants and confiscated the bodies of two others, the U.S. military said. The military said that one of the insurgents was caught as he tried to enter the Oprawi Hotel on the banks of the Tigris River and that the 12 others were arrested in a raid at a butcher shop believed to be harboring insurgents.

In other developments, the Central Criminal Court convicted two men on charges of possessing illegal weapons and sentenced each to four years in prison. U.S. soldiers said nearly 200 mortar rounds, artillery rounds, antitank rounds and mortar fuses had been found in the men's possession. Three brothers of the convicted men were found not guilty.

Special correspondents Saad Sarhan in Najaf and Bassam Sebti and Omar Fekeiki contributed to this report.

Followers of Moqtada Sadr patrol the streets of the Sadr City section of Baghdad, where the cleric's militia clashed with U.S. forces.