US helicopters circled above Najaf cemetery, while heavy gunfire and mortar rounds boomed across the Shia holy city.

A US soldier was killed Thursday around Iraq's central holy city of Najaf, where fierce fighting pitted US troops against Shia Muslim militiamen, the US military said.

The soldier was killed and five others wounded when their convoy came under rocket-propelled grenade attack and gunfire near Najaf, said a statement.

The clashes continued into the late afternoon after a brief lull following the downing of a helicopter attached to the 11th Marine Expeditionary unit.

"A UH-1 marine helicopter was shot down at about 11:45 am (0745 GMT)," a military spokesman said. The two crew members were recovered alive, although the extent of their injuries was not immediately known, he added.

Najaf's general hospital, near the scene of the fighting in 1920 Revolution Square, came under rocket attack, killing a doctor and seriously wounding four other staff members, said a health ministry official.

In all, medics said four people were killed and 19 wounded in the clashes, which erupted overnight when Shia radical leader Moqtada Sadr's Mehdi Army militia attacked Iraqi police.

And in what could be a sign of a fresh coordinated onslaught against foreign troops, Sadr's representative in Iraq's second city of Basra declared holy war against British forces after four of their comrades were arrested.

Three militiamen were killed and another three wounded in a brief skirmish with British troops in northern Basra, Sheikh Saad al-Basri said.

"We will wage jihad (holy war) and war against the foreign troops, not against police and Iraqi forces," he vowed earlier.

"However, if they (the Iraqi personnel) fight on the side of the occupiers, we will strike them harshly."

A military spokesman said British troops came under three small arms fire attacks that caused no multinational casualties or damage to equipment.

Two projectiles were fired at a multinational base camp in Sibah, south of Basra on the Kuwaiti border, causing no harm, the military said.

In Sadr's Baghdad slum stronghold of Sadr City, another person was killed and two others wounded in fighting between US forces and the Mehdi Army, said officials at Al-Shuader hospital.

Back in Najaf, Sadr aides accused US troops of damaging a minaret of the city's holiest shrine, the mausoleum of Imam Ali, which has remained a Sadr stronghold since his militia waged an uprising against US forces in the spring.

"The occupation forces and those collaborating with them attacked the Imam Ali mausoleum and we call on Muslims to help defend this holy place," his fighters shouted over loudspeakers.

The US military branded the overnight clashes an "overt violation" of a June truce which had quelled Sadr's uprising.

Interior Minister Falah al-Naqib told a Baghdad news conference that the government would not negotiate with the militias.

"We are not going to go into any negotiations. We are going to fight these militias. We have enough power and enough strength to stop and kick those people out from the country," he said.

He said Iraqi security forces had captured "many groups" linked with alleged Al-Qaeda operative Abu Mussab al-Zarqawi and "other movements aimed at destroying Iraq".

Tensions have run high in ravaged Najaf -- once a jewel in the crown of a vibrant Iraqi pilgrimage industry -- despite the June ceasefire agreement that allowed police forces to resume control of much of the city.

The truce also failed to address the fate of the Mehdi Army and the status of an arrest warrant issued against Sadr in connection with the murder of a rival cleric last year.