The US military has started flying attack helicopters against Islamic State militants in Iraq for the first time, officials said Monday, marking an escalation in the air war that puts American troops at higher risk.

US Central Command, which is overseeing the air campaign in Iraq and Syria, said helicopters took part in strikes on Sunday and Monday in Iraq, at a time when Iraqi government forces are struggling against IS fighters in the country's west.

"It has a capability that was asked for by the Iraqi government," said spokesman Major Curtis Kellogg.

"It was a capability that was available, that was requested, and it was a fit" for the targets to be attacked, he told AFP.

Flying at a lower altitude and at a slower speed than fighter jets and bombers, the helicopter gunships are more vulnerable to hostile ground fire.

The higher risk for troop casualties comes despite President Barack Obama's pledge not to send a ground force to fight the war, promising "no boots on the ground."

The move also raised questions about the effectiveness of the air war so far, which began in Iraq on August 8 and was extended into Syria on September 23.

Officials said privately the helicopters were Apache aircraft, which can linger over targets and strike at close distance with heavy fire power.

"It's a natural progression," said a US defense official, who spoke on condition of anonymity. In a fluid battle, helicopters can offer the best option instead of "fast movers."

But he added: "It's more vulnerable. No question about it."

Some of the most deadly incidents for US forces in Afghanistan resulted from helicopters being shot down by insurgents.

Central Command did not say where in Iraq the helicopters hit at the IS group.

On Sunday, the helicopters were part of a mix of fighter and bomber jets that carried out a total of six raids in Iraq, it said in a statement.

On Monday, the choppers joined unmanned drone aircraft in conducting three strikes, two in Fallujah and one west of Ramadi, Centcom said.

While the air campaign has been backed by both parties for the most part in the US Congress, some lawmakers on the left have raised concerns the mission could expand and trigger a larger role for combat troops.

Representative Barbara Lee's spokesman told AFP that "the helicopter operations are part of her larger concern about US military involvement in the Middle East -- given that the American people are war-weary and this action has not been debate or authorized by Congress."

Spokesman James Lewis said that from the start Lee "has been gravely concerned with mission creep and risks to American servicemen and women."