NAJAF, Iraq - A radical cleric whose loyalists battled U.S. troops for the fifth straight day vowed today to fight to the death, and a suicide attacker detonated a car bomb northeast of the capital, killing six people and wounding the deputy governor who was the intended target, officials said.

Explosions and gunfire were heard throughout Najaf and U.S. helicopters hovered overhead as U.S. forces tried to drive Muqtada al-Sadr's Mahdi Army militiamen from a vast cemetery they have repeatedly used as a base. A U.S. tank rolled within 400 yards of Najaf's holiest site, the Imam Ali Shrine, also held by militiamen.

A Najaf hospital spokesman said three were killed, including two policemen, and 19 wounded. The U.S. military says hundreds of militants have been killed in the violence in recent days; the militiamen put the number far lower.

Al-Sadr vowed to keep up the battle.

"I will continue fighting," al-Sadr told reporters. "I will remain in Najaf city until the last drop of my blood has been spilled."

Iraq's defense minister, Hazem Shaalan, accused neighboring Iran of helping arm the Shiite militiamen.

"There are Iranian-made weapons that have been found in the hands of criminals in Najaf who received these weapons from across the Iranian border," Shaalan said in an interview with the Arab-language television network al-Arabiya.

Iran has previously denied interfering in Iraq.

Government officials have said many of those involved in the Najaf violence were criminals and implied they were not true followers of the popular Shiite firebrand. But al-Sadr said the militants were his followers and described them as volunteers fighting for a cause.

"These are honest attacks against the occupation," he said, referring to the U.S. troop presence in the country.

"Resistance will continue and increase day by day," he said. "Our demand is for the American occupation to get out of Iraq. We want an independent, democratic, free country."

Al-Sadr's words were a challenge to interim Prime Minister Ayad Allawi, who visited the war-shattered city Sunday under heavy security and called on the Shiite militants to stop fighting.

"We think that those armed should leave the holy sites and the (Imam Ali Shrine compound) as well as leave their weapons and abide by the law," he said.

Much of the fighting has centered on the vast cemetery near the Imam Ali Shrine. U.S. forces using helicopter gunships launched a renewed offensive Sunday to drive militants out of the cemetery after claiming two days earlier to have secured the area in some of the fiercest fighting.

"We cannot conduct negotiations under shelling," al-Sadr said.

Mahdi Army militiamen in Baghdad kidnapped a senior Iraqi policeman, Brig. Raed Mohammed Khudair, who is responsible for all police patrols in eastern Baghdad, said Col. Adnan Abdel Rahman, an Interior Ministry spokesman.

In a video broadcast on Al-Jazeera television, militants demanded the government release all Mahdi Army prisoners in exchange for Khudair, whom they snatched Sunday.

Iraq's Interior Ministry clamped a curfew today on Sadr City, a Shiite neighborhood in eastern Baghdad where U.S. troops and al-Sadr militiamen have also been fighting.

In the southern city of Basra, masked al-Sadr followers patrolled some main streets today and set up checkpoints. No Iraqi police or British troops could be seen, witnesses said.

The Mahdi Army threatened to take over local government buildings in Basra if U.S. troops did not leave Najaf, and also said they would target oil pipelines and ports in southern Iraq.

Also today, the military reported that a U.S. Marine was killed in action Sunday in the western province of Anbar.

The death brought to at least 927 the number of American service members who have died in Iraq.