Militant cleric Muqtada al-Sadr (search), whose Shiite militia has been battling U.S. forces across Iraq, warned Monday that he would fight "until the last drop of my blood has been spilled," in his first appearance since the violence began.

The five-day-old uprising by al-Sadr's Mahdi Army began to affect Iraq's crucial oil industry, as pumping to the southern port of Basra (search) -- the country's main export outlet -- was halted because of militant threats to infrastructure, an official with the South Oil Company said.

About 1.8 million barrels per day, or 90 percent of Iraq's exports, move through Basra, and any shutdown in the flow of Iraq's main money earner would badly hamper reconstruction efforts. Iraq's other export line -- from the north to Turkey -- is already out of operation.

Clashes intensified around Basra, where a British soldier was killed and several others wounded in fighting with militia near the cleric's office Monday, the British Ministry of Defense said. Three militants were killed and more than 10 others wounded, a senior Iraqi police official said.

In the holy city of Najaf, the main scene of fighting, U.S. forces tried once more to drive militiamen out of a sprawling cemetery, and an American tank rattled up to within 400 yards of the Imam Ali Shrine (search), Najaf's holiest site, which fighters have reportedly been using as a base.

While U.S. and Iraqi forces were trying to quell the eruption of Shiite violence, attacks by Sunni Muslim militants persisted around Baghdad: A homicide car bombing targeting a deputy governor killed six people, and a roadside bomb hit a bus, killing four passengers.

The U.S. military also said a U.S. Marine was killed in action Sunday in the western province of Anbar, a hotbed of Sunni militancy. The death brought to at least 927 the number of American troops who have died in Iraq.

An insurgent group warned in a videotaped message it would launch a campaign of attacks on government offices in Baghdad, telling employees to stay away. Al-Sadr's militants also kidnapped a top Baghdad police official and demanded that their comrades in detention be freed.

In the city of Nasiriyah, 190 miles south of Baghdad, militants raided the local office of interim Prime Minister Ayad Allawi's Iraqi National Accord party, set it on fire and warned party members to leave the city, an assault captured on video obtained by Associated Press Television News.

One of the attackers in the video denounced Allawi as "subservient to the occupation." There were no injuries in the Sunday night attack, said police Capt. Haydar Abboud.

Al-Sadr's vow to keep fighting was a defiant challenge to Allawi, who visited Najaf on Sunday and called on the Shiite militants to stop fighting.

"I will continue fighting," the young, firebrand cleric told reporters in Najaf. "I will remain in Najaf city until the last drop of my blood has been spilled."

"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."

President Bush said Monday that coalition forces were "making pretty good progress about stabilizing Najaf."

Explosions and gunfire rattled the city, as fighting remained centered on the vast cemetery near the Imam Ali Shrine. The U.S. military said Mahdi Army gunmen were launching attacks from the cemetery and then running to take refuge in the shrine compound, one of the holiest sites in Shia Islam.

Najaf Gov. Adnan al-Zarfi gave U.S. forces approval to enter the shrine, a senior U.S. military official said Monday. "We have elected at this point not to conduct operations there, although we are prepared to do so at a moment's notice," the official said.

Such an offensive would almost certainly cause widespread outrage among the nation's Shiite majority and further exacerbate the crisis.

The military official estimated that 360 insurgents were killed from Thursday, when fighting began, and Sunday night, a figure the militants dispute. Five U.S. troops have been killed in the fighting. About 20 police also have been killed, Najaf police chief Brig. Ghalib al-Jazaari said.

Hospital officials in Najaf said four people, including three policemen, were killed Monday and 19 others injured. In addition, 13 previously unidentified bodies had been brought to the hospital.

The fighting has shattered a series of delicate truces worked out two months ago that ended the Mahdi Army's first uprising, which broke out in April. During that period, U.S. commanders vowed to "capture or kill" al-Sadr, but later tacitly agreed to let Iraqi authorities deal with the cleric.

U.S. forces were apparently continuing the hands-off policy toward al-Sadr. The senior U.S. military officer in Baghdad, speaking on condition of anonymity, said the cleric "is not an objective; we are not actively pursuing him."

But the fighting has complicated the security situation for Allawi's government as it tried to take a tough stance against the mainly Sunni campaign of attacks, bombings and shootings plaguing Iraq for the past 15 months.

In a sign of the deterioration of the situation in Najaf, the Polish military returned command in the province and neighboring Qadisiyah province to the U.S. Marines. The Poles had received command in the two provinces only 10 days ago.

The government on Monday imposed a nighttime curfew in Baghdad's Shiite neighborhood of Sadr City, the scene of heavy battles in the past few days.

Mahdi Army militiamen in Baghdad also 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 the pan-Arab network Al-Jazeera, militants said the government should release all Mahdi Army prisoners in exchange for Khudair.

Al-Jazaari, the Najaf police chief, accused al-Sadr's men of kidnapping one of his relatives -- Zaid Khalaf, also a police officer -- beheading him and burning his body.

During the five days of fighting in Najaf, police had arrested 300 militants, he said.

Meanwhile, two Jordanian and two Lebanese hostages were freed from captivity in Iraq on Monday, according to relatives. All four were truck drivers. The Jordanians were held captive for two weeks; the Lebanese for a week.

The car bombing Monday in Balad Ruz, 40 miles northeast of Baghdad, occurred about 7:30 a.m. when a white station wagon laden with explosives blew up outside the home of Aqil Hamid al-Adili, deputy governor of Diyala province, military spokesman Maj. Neal O'Brien said.

Six people, including four police officers, were killed and 17 others wounded in the attack, including al-Adili and his 9-year-old son, police Brig. Daoud Mahmoud said.

The Tawhid and Jihad group linked to the Jordanian militant Abu Musab al-Zarqawi claimed responsibility.

An audio tape purportedly made by the spiritual leader of the group, Sheik Abu Anas al-Shami, warned Monday of attacks on Allawi and police and military officials.

"We will not allow you to destroy our hopes in this blessed holy war," the speaker says in the tape that appeared on an Islamic Web site known as a clearinghouse for statements by the group.

"As for you Allawi ... we have prepared a potent poison and a sharp sword." The tape also accuses Iraqi soldiers and police of repeating the "hideous crime" of working for tyrants.

The voice could not be immediately verified as that of al-Shami, but the cleric did issue a similar pro-militant tape on the same Web site on July 28.

In other violence, a roadside bomb blew up next to a bus traveling on the main street in the town of Khalidiya, west of Baghdad, killing four passengers and wounding four people.

Mortars apparently targeting the fortified Green Zone, which houses the U.S. Embassy and Iraqi government offices, exploded in Baghdad early Tuesday. Most appeared to fall short.