US forces in Iraq have launched air strikes on positions held by supporters of radical Shia cleric Moqtada Sadr in the holy city of Najaf.
Militants have been battling US forces for a week
Plumes of smoke rose from the city's massive cemetery as the aircraft opened fire on Tuesday, witnesses said.
The air raids came after US troops, speaking Arabic through loudspeakers, urged civilians to evacuate Najaf.
The Red Crescent says it is trying to get supplies into the general hospital, which is "stretched to the limits".
Spokeswoman Nada Doumani, in Jordan, told the BBC it was hard to get accurate reports from Najaf, as "the city is almost totally closed, [it is] very hard to move around. The governor of Najaf has apparently urged the people to leave the city centre".
The BBC's Alastair Leithead in Baghdad says the Najaf stand-off is the biggest test for Iraq's interim government.
Artillery and tank fire was reported on Tuesday around Najaf's vast cemetery and close to the Imam Ali shrine.
The shrine - one of Shia Islam's holiest sites - was used for a defiant news conference by Mr Sadr on Monday.
A spokesman for interim Prime Minister Iyad Allawi said the US-led forces were answering a request to help Iraqi police and national guardsmen.
NAJAF'S BLOODY RECENT TIMES
August 2003: At least 95 people, including leading Shia politician, killed in car bomb
April 2004: US-led troops launch offensive against Sadr forces, starting clashes that will continue until June
4 June 2004: Truce agreed between Sadr and US-led forces, though sporadic fighting continues
5 August 2004: Ceasefire collapses
One of Najaf's leading figures, Shia cleric Ayatollah Ali Sistani, in London for heart treatment, urged an end to the uprising, which he said gave US forces an excuse to intervene.
His spokesman told the BBC he "stresses that everyone should comply with the rule of law, and no-one should take the law into his own hands".
Najaf was plunged back into violence last Thursday when a previous ceasefire between foreign forces and the Shia militia collapsed after six weeks.
As well as being a key military battleground, correspondents say it is also at the heart of an intense political stand-off.
Mr Allawi delivered a tough message during a surprise visit to the city on Sunday, ordering insurgents to quit fighting.
But on Monday, Mr Sadr said he would defend the city "until the last drop of my blood".
Each side is blaming the other for the fighting. The number of casualties is unclear.