American ground troops could be deployed against Islamic State and the Levant (ISIL) fighters in Iraq, according to the most senior US military officer.

General Martin Dempsey, chairman of the US military's joint chiefs of staff, said on Tuesday he might recommend having US troops do more, potentially accompanying Iraqis during complicated offensives, such as a battle to retake the northern city of Mosul from ISIL fighters.

"It could very well be part of that particular mission - to provide close combat advising or accompanying for that mission," he said.

Dempsey acknowledged that President Barack Obama's "stated policy is that we will not have US ground forces in direct combat.

"But he has told me as well to come back to him on a case-by-case basis".

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The Obama administration also plans to carry out air strikes against the group's fighters in Syria and will target the ISIL's sanctuaries, command centres and logistic networks, Chuck Hagel, the US defence secretary, told the Senate Armed Services Committee.

"This plan includes targeted actions against ISIL safe havens in Syria, including its command and control, logistics capabilities, and infrastructure," Hagel said.

However, Dempsey said at the same hearing that the bombing would not resemble the large-scale raids that accompanied the start of the 2003 US-led invasion of Iraq.

Commanders at the time dubbed the campaign a bid to provoke "shock and awe" among deposed leader Saddam Hussein's troops.

"This will not look like 'shock and awe' because that is not how ISIL is organised, but it will be persistent and sustainable," Dempsey said.

The military leaders warned of a further escalation in their battle against ISIL just as two branches of the rival al-Qaeda group called for a united front against the war coalition the US is building.

US fighter jets have been targeting the group's fighters in northern Iraq since August 8, and in recent days hit its fighters southwest of Baghdad for the first time, in a significant expansion of the campaign.

The US strikes against ISIL fighters in the Sadr al-Yusufiyah area, 25km from Baghdad, was the first in support of Iraqi forces near Baghdad.

They bring the number of US air strikes across Iraq to 162. The CIA estimates that the ISIL may be able to field as many as 31,500 fighters - many of them foreign volunteers.

Obama has pledged to expand American efforts and US diplomats are rushing to put together an international coalition for a "relentless" campaign against ISIL.

The slow coming together of this alliance drew a fierce reaction from al-Qaeda's branches in Yemen and in North Africa, who said self-proclaimed jihadist forces must also unite against the common threat.

In a joint statement, Al-Qaeda in the Arabian Peninsula (AQAP) and Al-Qaeda in the Islamic Maghreb (AQIM) urged their "brothers" in Iraq and Syria to "stop killing each other and unite against the American campaign and its evil coalition that threatens us all".

Source: Agencies