WASHINGTON - The United States is using Apache helicopters in the battle to retake Iraq’s second city of Mosul after more than two years of Islamic State group rule, the Pentagon has said.

The US military, backing the ground campaign by Iraqi forces, is directing the attack helicopters against explosives-packed vehicles the jihadist group is employing for suicide bombings, Pentagon spokesman Peter Cook said on Monday.

The helicopters are being used "with significant effect" in Mosul, he said.

"We anticipate that this nimble and precise capability will continue to enable Iraqi progress in what we expect will be tough fighting to come," Cook added.

However, few helicopters are being used, with officials suggesting that the number of choppers is in the single digits.

Before their deployment to Mosul, the US military employed Apaches very occasionally for combat operations against the jihadists, especially in the Tigris River Valley in June.

Their use reflects the increasing level of risk the Obama administration has had to accept to defeat the IS group in Iraq.

The helicopters fly at lower altitudes and slower speeds than fighter jets and bombers, making them much more vulnerable to enemy fire and increasing the risk of casualties.

President Barack Obama -- who as a senator in 2003 opposed the US-led invasion of Iraq launched by his predecessor George W Bush -- ruled out using American soldiers on the ground against the fighters in Iraq and Syria.

Faced with the difficulties the Iraqi army is confronting against the jihadists, however, he has adjusted his policy, bringing US ground forces closer to the fighting.

Special forces have been enabled to conduct raids to capture or kill IS group leaders.

US military advisers are also moving closer to the front line. - AFP