John Kerry has said that US support for Iraqi security forces will be "intense and sustained" to help them combat an armed uprising that has swept through the country's north and west.
The US secretary of state was speaking in the Iraqi capital, Baghdad, where he is meeting the country's leaders and urging Nouri al-Maliki's Shia-led government to give more power to political opponents before the Sunni fighters seize more control across the country.
"It is a moment of decision for Iraq's leaders," Kerry said on Monday.
"Iraq faces an existential threat and Iraq's leaders have to meet that threat."
Kerry said that during Monday's talks, Maliki reaffirmed his commitment to a July 1 date for forming a new government.
On Sunday Kerry said that the US would not pick or choose who rules Iraq. He said, however, the US had noted the dissatisfaction among Kurds, Sunnis and some Shia with Maliki's leadership and emphasised that the US wanted Iraqis to "find a leadership that was prepared to be inclusive and share power".
The US government has ruled out sending ground troops to aid the Iraqi government against its fight with the Islamic State of Iraq and the Levant (ISIL) - an al-Qaeda breakaway group - and its Sunni allies, but has kept an option open to use air power.
It has moved a carrier strike group and troops transports into the Arabian Gulf area.
The visit came as Sunni armed groups led by ISIL fighters have expanded their offensive in Iraq, capturing more territory from the government.
ISIL was reported on Sunday to have taken the towns of Qaim, Rawah and Anah in Anbar province. Qaim, located on the border with Syria, hosts a key crossing between the two countries.
Fighters also claim to be in full control of the northern city of Baiji, which hosts Iraq's biggest oil refinery, though the military denies that the group controls the refinery itself.
The Associated Press news agency, citing Iraqi military officials, reported that Sunni fighters captured two border crossings, the Turaibil crossing with Jordan and the al-Walid crossing with Syria, on Sunday.