BAGHDAD (Reuters) - Fierce fighting broke out in Baghdad’s Sadr City slum late on Friday after a senior aide to Shi’ite cleric Moqtada al-Sadr was killed in the holy city of Najaf to the south.

Heavy gunfire erupted at around 11:00 p.m. (2100 GMT) in several parts of the slum, an east Baghdad stronghold of Sadr’s followers and home to 2 million people.

A Reuters correspondent said U.S. helicopters and jets were swooping overhead and several of the aircraft fired missiles. The number of casualties was not immediately known.

As the fighting raged, loudspeakers on mosques blared out speeches in support of Sadr’s Mehdi Army fighters.

“We will not allow the Americans to enter the city whatever happens, if we lose our lives and our sons,” they said.

In Najaf, police set up road blocks and drove through the city with loudspeakers ordering shops closed and people off the streets after senior Sadr aide Riyadh al-Nuri was gunned down.

A struggle for power among Shi’ites in the south has involved frequent assassinations over recent years. But the death of someone so close to Sadr risks inflaming those tensions at a time when his militia has been at the centre of an upsurge in violence in Baghdad and throughout the south.

Nuri’s sister is married to the cleric’s brother.

“This is the hand of the occupier and his successor reaching out traitorously and aggressively against our precious martyr,” Sadr said in a statement. “It is my vow that I will not forget this precious blood.”

U.S. Defense Secretary Robert Gates said Sadr would not be treated as an enemy if he played a peaceful political role.

“Those who are prepared to work within the political process in Iraq, and peacefully, are not enemies of the United States,” Gates told reporters in Washington.

Much of the fighting this week has centered on Sadr City, where Sadr’s Mehdi Army gunmen have faced U.S. and Iraqi forces that have surrounded the slum and blockaded it for two weeks.

Huda Jassim, a student, said Friday night’s fighting appeared to be the worst since last Sunday, when Iraqi and U.S. forces launched an offensive into parts of the slum that triggered a week of battles.

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One band of six Mehdi Army fighters could be seen heading to battle with rocket propelled grenades and heavy machine guns.

PRAYER

Inside her house, teacher Salwa Naser, 24, recited prayers as the sound of fighting echoed: “My God, my God, make their fire against us like water,” she prayed.

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Iraqi police said militants had attacked a U.S. military base in Shaab, northwest of Sadr City, late on Friday with mortars, rocket-propelled grenades and rifles, triggering fighting there.

U.S. tanks were being deployed amid clashes in Sadr City and a roadside bomb had hit an American patrol in the New Baghdad, another part of east Baghdad, the police said.

President Jalal Talabani, a Kurd, reached out to the Sadrists, condemning Nuri’s shooting and meeting two members of parliament from Sadr’s bloc.

“They discussed the importance of quieting the situation and solving disputes peacefully and through talk and political means,” Talabani’s office said in a statement.

U.S. and Iraqi forces have clashed with Sadr’s Mehdi Army since late March, when Prime Minister Nuri al-Maliki launched a crackdown on the militia in the southern city of Basra.

There have also been renewed clashes in Basra. A U.S. aircraft launched an air strike that killed six people and wounded one after Iraqi troops were fired upon early on Friday morning, said British Major Tom Holloway, a spokesman for U.S. and British forces in the south.

U.S. forces say militants in Sadr City have been responsible for rocket and mortar strikes across the capital, including at the heavily fortified Green Zone diplomatic and government compound in the city centre.

A missile ripped a hole in the second floor of the landmark Palestine Hotel across the Tigris River from the Green Zone on Friday, killing three civilians outside the hotel, police said.

Maliki has threatened to exclude Sadr’s movement from participating in provincial elections later this year unless he disbands his militia and turns over weapons.