Coalition troops have killed about 300 militants loyal to radical Shiite cleric Muqtada al-Sadr (search) in Najaf during the past two days in a continued attempt to quash rebellions by the cleric's loyalists in various Iraqi cities, the U.S. military said Friday.

Battles in other Shiite areas of the country have killed dozens more, according to Iraqi authorities.

The U.S. military also said two U.S. Marines and one soldier were killed in fighting in Najaf on Thursday and 12 troops were wounded. Fifteen U.S. soldiers were wounded in Baghdad.

Members of al-Sadr's Mahdi Army (search) militia seized four police stations in Amarah, 180 miles southeast of Baghdad, witnesses said.

Meanwhile, Lebanon's state news agency reported four Lebanese truck drivers were taken hostage by insurgents on a highway between Baghdad and Ramadi. It wasn't clear when the men were seized, but a Lebanese official said earlier in the day that they hadn't been heard from for 24 hours.

And Iraq's leading Shiite cleric, a voice of moderation since the fall of Saddam Hussein, flew to London for treatment of a heart condition.

Grand Ayatollah Ali al-Sistani's (search) advisers had warned that fighting in his base of Najaf might worsen his condition. Friday's trip was the first time in several years that Sistani had left Iraq.

Some of the worst fighting Friday took place in the Baghdad slum of Sadr City (search), where the Health Ministry said 19 people were killed and 111 wounded during fighting over the past two days between U.S. troops and al-Sadr militants. Separate attacks blamed on al-Sadr's followers wounded 15 American soldiers in Baghdad.

Friday marked the second day of fighting that has shattered truces to end a widespread rebellion two months ago.

The fighting began Thursday in the holy city of Najaf and has since spread to other areas across the country, and dozens have been reported killed and wounded.

Helicopter gunships pounded militant positions in fierce fighting in Najaf, while Italian soldiers exchanged gunfire with militants who attacked their positions and a police station in the southern city of Nasiriyah, an Italian military spokesman said. Clashes were also reported Friday between U.S. troops and insurgents north of the capital in Samarra.

In April, the Mahdi Army militia launched sustained attacks on U.S. and coalition troops in several cities, the first major Shiite violence against the Americans. The confrontation dragged on for two months until Iraqi politicians and religious leaders worked out a series of truces.

Each side blamed the other for breaking the ceasefire. The U.S. military accused the militants of repeatedly attacking police in Najaf, and al-Sadr loyalists accused U.S. forces of surrounding the cleric's house on Monday.

"We hold the American troops and the governor of Najaf totally accountable for the state the crisis has reached to now," Mahmoud al-Sudani, a spokesman of al-Sadr in Baghdad, told reporters. He called on the United Nations and Iraq's interim government to stop the violence.

"From our side we did not want to escalate the situation, because the situation in Najaf affects that of other Shiite areas," he said. "But the actions of the American troops have enraged the sons of these cities."

In Najaf, 100 miles south of Baghdad, U.S. helicopters on Friday attacked militants hiding in a cemetery near the Imam Ali Shrine in the old city at Najaf's center, where smoke could be seen rising. AC-130 gunships, helicopters and F-16s were patrolling the skies, while tanks and armored vehicles were reported moving into the city.

Iraqi policemen advanced toward the area, witnesses said. The streets were otherwise deserted and shops were closed.

"The area near the [Imam Ali Shrine] is being subjected to a war," said Ahmed al-Shaibany, an official with al-Sadr's office in Najaf. "Najaf is being subjected to ... total destruction," he said.

"We call on the Islamic world and the civilized world to save the city."

The U.S. military has accused the militants of hiding in the shrine compound to avoid retaliation by U.S. forces. It had no comment on Friday's clashes.

Battles between the two sides in Najaf have killed at least 10 people and wounded 40 others, according to Hussein Hadi of Najaf General Hospital official. The U.S. military said Thursday it had lost one soldier in the battle, killed seven militants and detained dozens of people. A Marine helicopter was also downed.

In Samarra, 60 miles north of the capital, guerrillas attacked a convoy of 10 U.S. Humvees at dawn, witnesses said. U.S. helicopters fired rockets at insurgent positions, and the U.S. convoy pulled out.

U.S. troops have launched "Operation Cajun Mousetrap" there, conducting a series of raids and patrols.

Ahmed Jadou'a, an official at Samarra Hospital, said at least two people were killed and 16 injured during the fighting. Two houses were also destroyed.

About 230 miles south of Baghdad in Amarah, Al-Sadr's army was seen roaming the streets there with guns and rocket-propelled grenades. A British base there got pounded by more than 20 mortars Thursday but no one was wounded.

In the southern city of Nasiriyah, assailants attacked Italian troops with automatic weapons, an Italian military spokesman said on condition of anonymity. They also attacked a police station, prompting the local governor to call for Italian military assistance, he said. There were no coalition casualties, the spokesman said.

The fighting, which lasted until dawn Friday, killed eight Iraqis, including five militants, and injured 13 others, according to AbdelKhuder al-Tahir, a senior Interior Ministry official.

"Today, the city is more stable. Policemen and National Guard are in control of government buildings one side of the city, while Italian forces are in control of the other side. Some of al-Sadr's followers are moving in the center of the city, but the rest of the city is under our control," he said.

Another Italian spokesman in Nasiriyah, Cpt. Ettore Sarli, said the assailants were al-Sadr militants. In another incident there, Italian forces repeatedly fired at a car that failed to stop at a roadblock. The car, believed filled with explosives, then blew up, said Sarli, giving no casualty figure.

A coalition base near Najaf, Camp Golf, was hit by mortar fire early Friday, while rounds fired at a base housing Ukrainian troops missed their target, a Polish military spokesman said. No one was hurt.

Tensions were also running high in the southern city of Basra, where British troops clashed Thursday with the Mahdi Army. Violence there killed five al-Sadr fighters, said As'ad al-Basri, an al-Sadr official in the city.

"The clashes with the British will continue and they are going to escalate after Friday prayers," he said.

FOX News' Caroline Shively and The Associated Press contributed to this report.