"We are at war with ISIL, as we are with al Qaeda," Hagel told the US Senate's Armed Services Committees, using a different name for the terrorist group "Islamic State" (IS). Hagel warned that "if left unchecked, ISIL will directly threaten our homeland and our allies."

Hagel added that the planned US airstrikes against 'IS' jihadists in Syria would target extremist sanctuaries, command centers and logistics networks.

But the US military's top-ranking officer, the chairman of the joints Chiefs of Staff General Martin Dempsey, who also testified on Tuesday, stressed that the bombing would not resemble the large-scale raids at the start of the US-led invasion of Iraq in 2003.

"This will not look like 'shock and awe' because that is not how ISIL is organized, but it will be persistent and sustainable," Dempsey said, referring to the name often used to describe the strikes on Iraq at the time of the invasion.

Hagel said the action in Syria would be a "long-term effort, but we will do it right," and that the US would aim to train and equip forces, and give them tactical and strategic guidance, to create what he referred to as "legitimate forces" in Syria.

Ground forces?

When pressed by lawmakers during questioning, Dempsey said he "would go back to the president and make a recommendation that may include the use of ground forces."

He told a Senate panel that the goal for American advisers is to help Iraqi forces with planning, logistics and coordinating military efforts by coalition partners to take out members of IS.

"To be clear, if we reach the point where I believe our advisers should accompany Iraqi troops on attacks against specific ISIL targets, I will recommend that to the president," he said.

The US military has already conducted strikes near Baghdad against 'IS,' which has seized large swaths of Iraq and Syria. Obama is seeking congressional support to train and equip vetted Syrian moderates fighting both the Islamic militants and forces loyal to President Bashar Assad.

But many lawmakers in an increasingly war-weary US Congress doubt that it will be possible to identify these moderates, and train them adequately, pointing to the largely unsuccessful effort to train Iraqi forces after the 2003 invasion. Some also questioned the government's resolve to go through with its plans for Syria and Iraq.

ng/rc (AP, AFP)