BAGHDAD, (Reuters):

All seven United States (U.S.) troops aboard a military helicopter were killed when it crashed near Baghdad yesterday, the fifth such aircraft to be lost in Iraq in less than three weeks, a U.S. defence official said.

"I can tell you that the initial report was seven," the official in Washington said, speaking on condition of anonymity.

The official did not say what caused the crash, but Iraqi witnesses reported seeing the helicopter in trouble during gunfire from the ground.

Five such aircraft have been lost in Iraq in nearly three weeks, killing 28 U.S. servicemen and private security contractors. The U.S. military said on Sunday it was adjusting its tactics after four helicopters had been shot down.

The high number lost in such a short time has raised questions about whether militants have changed tactics or are using more sophisticated weapons.

The self-styled Islamic State in Iraq, linked to al Qaida, claimed responsibility, saying the helicopter was shot down by what it described as its "air defence brigade".

"We tell the enemies that these (operations) are proof that God is giving us victory," the group said in a statement posted on a website used by Sunni militant groups.

Meanwhile, the U.S. military in Iraq announced that the new Baghdad security operation had officially begun, but would be rolled out only gradually.

"The implementation of the prime minister's plan has already begun and will be fully implemented at a later date, having all the parts and pieces he wants," U.S. military spokesman Major-General William Caldwell told reporters in Baghdad.

The crackdown is regarded as a last-ditch attempt to prevent all-out civil war between politically dominant Shi'ites and minority Sunni Arabs.

Iraqi government spokesman Ali al-Dabbagh said preparations for the Baghdad offensive were still being worked out.