A number of mortars have hit Baghdad's Green Zone, with eyewitnesses reporting smoke rising from the area around the US embassy.

About 10 mortars landed in the heavily fortified district where the government and parliament are based, and which is also home to many foreign embassies.

There were no immediate reports of casualties or damage.

It comes a week after Prime Minister Nouri Maliki warned militants against attacks on the Green Zone.

US military helicopters took to the air moments later from the Green Zone apparently searching for the firing positions.

Correct range

The Green Zone is supposed to be a safe area of operation for the US civilian and military authorities in Baghdad, US contractors and the post-Saddam Hussein Iraqi government.

In a recent report, the United Nations underlined the increased risks of operating in the area. According to UN figures, 26 people were killed by indirect fire from mid-February to late May.

The heavily-armored area of the Iraqi capital has been the target of increasingly frequent attacks.

However, rocket fire and mortar attacks are often ineffective as helicopter patrols mean that attackers have no time to find the correct range, with most attacks falling on open ground.

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Iraq's Prime Minister Nouri Maliki