As American troops took their fight with Shiite Muslim rebels to the center of the insurgent stronghold of Najaf on Tuesday, heavy projectiles damaged the facade of the Imam Ali shrine, among the holiest sites in Shiite Islam, a strike that could inflame anger against the United States.

U.S. military officers denied their forces caused the damage to the shrine, where blasts chipped a door frame that leads to the shrine's inner sanctum and put a hole in a nearby wall. In Baghdad, Army Brig. Gen. Mark Kimmitt, the chief U.S. military spokesman in Iraq, said rebels were the likely culprits, hoping "to provoke outrage so they could blame it on American forces."

In Najaf, a spokesman for the rebel cleric Moqtada Sadr blamed U.S. forces and appealed to Muslims to attack them. "Is it logical that a Muslim, Arab or Iraqi man can surrender to the infidel occupier?" said Qais Khazali, the spokesman. There were no reports of violence in other Shiite towns or mass reaction by the population of Najaf itself.

The question of who damaged the mosque has political as well as emotional significance. Several Shiite groups that oppose Sadr and either cooperate with or tacitly accept U.S. authority in Iraq would come under pressure to distance themselves from the Americans if U.S. forces were found to have caused the damage. Losing that support could cripple U.S. efforts to pacify Iraq.

To date, Grand Ayatollah Ali Sistani, the chief Shiite religious authority in Iraq, has avoided criticizing U.S. military moves against Sadr. But a word of opposition from Sistani, Iraqis say, would set off an explosion of anti-American sentiment among Iraq's majority Shiite population.

On Tuesday, his office issued a statement that denied reports he had blamed damage to the shrine on Sadr. "We don't have any information about which side targeted the holy shrine," the statement said.

The U.S. strike into the city center was part of an increasingly aggressive campaign to corner Sadr. For the past two days, infantrymen with the Army's 1st Armored Division riding Humvees and Bradley Fighting Vehicles have been penetrating deeper and deeper into Najaf, one of two adjacent cities where Sadr's forces dominate the streets.

Sadr has defied U.S. demands to surrender and disband his militia, the Mahdi Army. The Americans have said they want him to stand trial in Iraqi courts for his alleged role in the April 2003 killing of moderate cleric Abdel Majid Khoie. Sadr's arrest would take off the street a violent opponent to U.S. plans to transfer limited authority to a new Iraqi government on June 30.

During the past several weeks, U.S. troops have quelled Sadr-inspired uprisings in the towns of Kut and Diwaniya, while British forces have tamped down unrest in Basra and Amarah. U.S. forces have clashed inconclusively off and on for two weeks with Sadr's forces in the sprawling Baghdad slum of Sadr City, named after the cleric's late father.

Over the weekend, the Mahdi Army abandoned positions in Karbala, another Shiite holy city, where they had fought two weeks of tough battles against 1st Armored Division forces. On Tuesday, U.S. troops badly damaged the Mukhaiyam mosque in Karbala, which had been used by militiamen to store arms, according to images broadcast by al-Arabiya, a satellite television network.

The fighting in Najaf on Tuesday raged throughout the morning. At least eight insurgents were killed, three of them a few yards in front of the shrine's outer wall, witnesses said. Others were killed after American troops trapped them in a pair of abandoned police stations. Shooting also erupted at a vast cemetery northeast of the shrine where guerrillas have hidden among the tombstones and fired on U.S. patrols on the city's outskirts.

When combat subsided, a few hundred demonstrators gathered on the mosque grounds, wailed, wept, pounded their heads and shouted "Yes, yes, to Sadr! America is the enemy of God." Blood streaked the large courtyard.

Hussein Husseini, a Sadr representative, told reporters at the mosque that U.S. rockets fired from a plaza a few blocks from the shrine hit the structure and courtyard. A mob cut short his words and cursed and threatened journalists who were present.

"What is the limit?" asked Hamed Asadi, a religion student and Sadr supporter. "Is it when the Americans enter the shrine? I call on all the youths to protect the shrine from the Zionist enemy."

"We pray to God to give Sadr victory," said Qasim Zeini, a young guerrilla who shouldered a rocket-propelled grenade launcher. "He is the only one who challenged the occupation."

Insurgent fighters say that in recent days they have been aided by sympathizers from other cities. "We didn't call for anyone to come, but some people wanted to be in Najaf and fight the occupation forces. The resistance is not only in Najaf, but all over Iraq," said Fuad Turfi, a Sadr spokesman.

Farazdaq Mousawi, a Mahdi Army commander, said that most of his militia's arms, including mortars and rocket-propelled grenade launchers, were looted from depots belonging to the ousted government of Saddam Hussein. "We get weapons from people all the time, too," he said. "What the Americans did today will increase resistance against them, and now more people will support us."

All around him, young masked fighters scurried from market stall to market stall and hid under awnings to avoid detection by U.S. air surveillance.

Many Najaf residents have continued to oppose Sadr, whose presence in the city has paralyzed tourism by pilgrims, a mainstay of the city's economy. "The Mahdi Army brought ruin to the city. We pray to God to get rid of them and their leaders," said Hashim Abid Ali, owner of the Huda Hotel.

"We brought this on ourselves. What is happening is controlled by Iran," said Hussein Hasan, a student, echoing a common suspicion that Sadr is a tool of people in the neighboring Islamic republic who want to destabilize Iraq.

Ahmed Jasim, another resident, blamed rebel Sunni Muslims in central Iraq for a continuing flow of arms into the city. "They say they have brought food into the city, but in fact, they brought weapons," he said. "Farms surround Najaf. We don't need any food."

In Baghdad, a car bomb exploded Tuesday, injuring five Iraqis and leaving the now-familiar sight of twisted, burning wreckage on a downtown street.

Later in the day, insurgents fired rockets from an apartment building toward a police station on Saadoun Street, one of Baghdad's main thoroughfares. It missed the station, but hit a second structure where two U.S. soldiers were posted, wounding them.

For the second time in a week, saboteurs blew up an oil pipeline, key to the export of Iraq's only significant foreign exchange earner. The conduit, which runs to the Turkish port of Ceyhan, was disrupted near the northern oil city of Kirkuk, according to oil executives in Baghdad. A similar explosion damaged a pipeline in the far south that takes crude to the Persian Gulf. In April, Iraq exported about 1.8 million barrels of oil a day. Both explosions will slow deliveries this month, U.S. officials said.

Williams reported from Baghdad.

The Imam Ali shrine in Najaf is considered the most sacred Shiite site in Iraq. U.S. forces pushed deeper into the city in an effort to corner radical cleric Moqtada Sadr, whose militia, the Mahdi Army, has controlled the city for weeks.Muslims weep at the gates to the Imam Ali shrine after it was damaged in fighting. The incident could erode attempts to quell a Shiite uprising.An Iraqi anguishes near bloodstains inside the Imam Ali shrine after the site was hit by projectiles. At least eight Iraqis were killed in the U.S. offensive.