US helicopters have attacked targets in the Iraqi holy city of Najaf in a second day of fighting between US-led coalition forces and Shia militiamen. US tanks and armoured vehicles are said to be moving into the city, where 10 people are reported killed. Columns of thick black smoke have been seen rising from Najaf, and power and telephone lines are reported cut. There have also been heavy clashes in the Sadr City area of Baghdad, where more than 30 have died since Thursday. Twenty-seven Iraqis died in the suburb on Thursday, and a further seven were killed on Friday. The US military reports that 16 of its troops have been wounded there. The militia, loyal to radical cleric Moqtada Sadr, has also been fighting Italian troops in Nasiriya, where four Iraqis are said to have died. A further two Iraqis are reported killed in clashes between US forces and insurgents in the city of Samarra. Mr Sadr had earlier called for a truce to be restored after a day of fighting between his Mehdi Army militia and US troops. The truce was agreed in June after a two-month uprising against the coalition led to violence across central and southern Iraq. The two sides blame each other for starting the latest outbreak of fighting. The latest clashes in Najaf, home to Shia Islam's holiest shrine, are being described as the worst fighting there since the truce agreement was reached. US helicopters fired rockets at the vast Valley of Peace cemetery, where militiamen loyal to Mr Sadr are based. And a correspondent for French news agency AFP said "dozens" of US tanks and armoured vehicles were entering the city. Medics in Najaf report 10 people dead and 34 wounded in Friday's violence. Thursday saw the US military report seven militiamen and one American soldier killed in Najaf, while a US helicopter was shot down, injuring two. Mr Sadr's aides accused US troops of damaging a minaret of Najaf's holiest shrine, the mausoleum of Imam Ali. They are calling on local Muslims to "take up arms to defend your holy places against the occupiers". Meanwhile, senior Iraqi Shia cleric Grand Ayatollah Ali Sistani is flying to London, where he is expected to have medical treatment for a heart condition. Grand Ayatollah Sistani, who lives in Najaf, has been receiving treatment from cardiologists, but his aides said they feared the current situation in the city could hamper his access to proper medical care.