US jets have bombed ISIL fighters south of Baghdad who had attacked Iraqi soldiers, in the first expansion of the campaign against the armed group.

The US Central Command early on Tuesday said the attack was launched after Iraqi forces called for assistance - the first time the US had used force with the sole intention of directly supporting Iraq troops fighting ISIL.

Previous US policy was to conduct air raids to protect US interests and personnel, assist Iraqi refugees and secure critical infrastructure.

Officials said the raid represented a broadened mission authorised by the US president, Barack Obama, to go on the offensive against ISIL wherever it is.

They said another US air raid on Tuesday targeted ISIL positions near Mount Sinjar, an area in the north of the country where ISIL besieged the Yazidi religious minority.

"These strikes were conducted under authority to protect US personnel and facilities, support humanitarian efforts, and help Iraqi forces on the offensive against ISIL terrorists," said a statement by the US Central Command.

In total, the strikes destroyed six ISIL vehicles near Sinjar and an ISIL fighting position southwest of Baghdad.

They bring the number of US air attacks in Iraq to 162 since US forces re-entered the country in August.

Earlier, a coalition of 30 countries including western and Arab nations promised to combat ISIL in Iraq and Syria, at a strategy conference in Paris. However, no coalition roles have been made public.

Obama has promised to degrade and destroy ISIL in areas it controls in northern Iraq and Syria.

On Sunday, US officials told the AP news agency that any US jets threatened by Syrian regime air defences would be authorised to act in self defence.