US plans to carry out air strikes against Islamic State and the Levant (ISIL) fighters in Syria and will target the group's sanctuaries, command centres and logistic networks, Pentagon chief Chuck Hagel has 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 told the Committee on Tuesday.

But the US military's top-ranking officer, General Martin Dempsey, told 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.

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Commanders at the time dubbed the campaign a bid to provoke "shock and awe" among ex-president 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.

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 Washington is building.

US warplanes 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 the capital.

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 vowed to expand American efforts and US diplomats are scrambling 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 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