Baghdad's heavily fortified Green Zone, which houses the Iraqi parliament and US embassy, was hit by a sustained barrage of rocket or mortar bomb fire early on Sunday, witnesses and officials said.

It was not immediately clear where most of the missiles landed or if there were any casualties after an apparent attack lasting about 15 minutes, Reuters witnesses said.

The US military has blamed past missile attacks on the Green Zone on rogue elements of anti-US cleric Moqtada al-Sadr's Mehdi Army militia. Sadr last month renewed a seven-month old ceasefire for his militia.

A large plume of thick black smoke could be seen rising from one area of central Baghdad's Green Zone, which houses many government ministries and diplomatic missions. Sirens could be heard warning people to take cover.

At least two US attack helicopters could be seen circling over an area in the Iraqi capital's northeast soon after the attack. A US government official, who asked not to be identified, said the Green Zone had come under "indirect fire", a term commonly used to described mortar or rocket fire.

There was no immediate response from the US military to telephone calls and emails about the attack. The US embassy routinely does not comment on such attacks for security reasons.

The barrage, which started just before 6 a.m., came in three separate volleys and lasted for a total of about 15 minutes.

The 10 sq km (4 sq mile) Green Zone, located on the left bank of the Tigris River that cuts through Baghdad, has often been targeted by mortars and rockets, although the frequency of attacks has diminished with improved security.

Sadr's ceasefire has helped contribute to a 60 percent fall in attacks across Iraq since last June, the US military says. - Reuters