The Pentagon chief described the ISIL militants as a serious threat only days after suggesting Washington was not on the verge of any military action to help the Iraqi government in its fight with the extremists.

"Make no mistake -- and this country should not make any mistake on this, nor anyone in Congress -- this is a threat to our country," Hagel said of the militants during a stop at a US naval submarine base in the state of Georgia.

"This is a force that is sophisticated, it's dynamic, it's strong, it's organized, it's well-financed, it's competent, " Hagel told troops in remarks broadcast by the Pentagon.

"And it is a threat to our allies all over the Middle East. It's a threat to Europe," he said at the base in King's Bay.

The extremist group "may not appear to be an imminent threat to the United States," but "it is a threat to the United States," Hagel said.

"It is a ... clear threat to our partners in that area, and it is imminent," he said, without elaborating.

After launching an offensive last month, the ISIL terrorist group has overrun areas in some provinces north and west of Baghdad, raising fears of an all-out sectarian war.

President Barack Obama has sent military advisors to Baghdad to help the Iraqi army counter the militants but has so far stopped short of ordering US air strikes against the militants.

US officials and experts fear the extremists, who are also fighting in Syria's civil war next door, will carve out sanctuaries that could serve as training camps and launching pads for terror attacks on the West.

Hagel said assessments of the Iraqi army by the roughly 200 US military advisors on the ground would be finished in "the next few days."

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