Jihadists who have seized Iraq's second city of Mosul pose a threat to the entire region, the United States warned Tuesday, voicing deep concern about the "extremely serious" situation.

Condemning militants from the Islamic State of Iraq and Levant (ISIL) "in the strongest possible terms," White House spokesman Josh Earnest called on Prime Minister Nuri al-Maliki and other leaders to do more to address "unresolved issues" to ensure they are governing "with the interests of all Iraqis in mind."

"It should be clear that ISIL is not only a threat to the stability of Iraq, but a threat to the entire region," said State Department spokeswoman Jen Psaki, in a statement.

She stressed that Washington backed "a strong coordinated response to push back against this aggression."

The militants launched a major assault on Iraqi security forces late Monday and grabbed Iraq's northern province of Nineveh, including its capital Mosul.

Jihadists also seized several areas in Iraq's Kirkuk province on Tuesday which borders Nineveh, an Iraqi police officer said.

ISIL was drawing "strength from the situation in Syria, from which it transfers recruits, sophisticated munitions, and resources to the fight in Iraq," Psaki said.

The most powerful militant group in Iraq, ISIL is a key force in the rebellion against President Bashar al-Assad in neighboring Syria.

US officials in Washington and Baghdad were "tracking events closely in coordination" with the Iraqi government, Psaki said.

A top US diplomat, deputy assistant secretary Brett McGurk, has been in Iraq since the weekend for talks on how to combat the ISIL threat.

"The United States stands with the Iraqi people and the people of Nineveh and Anbar now confronting this urgent threat," Psaki said.

"We will continue to work closely with Iraqi political and security leaders on a holistic approach to diminish ISIL's capacity and ability to operate within Iraq's borders."

The United States has already delivered some 300 Hellfire missiles to Iraqi forces as well as millions of rounds of small arms fire, and thousands of rounds of tank ammunition, helicopter-fired rockets, machine guns, flares and sniper rifles, Earnest said.

"This administration is committed to preserving the partnership that we have with the Iraqi government... There is some assistance that we can provide and have been providing, and we'll continue to do that."