The State Department's statement came after on June 11 fighters from the Islamic State of Iraq and the Levant (ISIL), an Al-Qaeda splinter group, seized the central city of Tikrit, one day after capturing Mosul, Iraq's second-largest city.
State Department spokeswoman Jen Psaki said Washington is committed to "working with the Iraqi government and leaders across Iraq to support a unified approach against ISIL's continued aggression."
Psaki said it's expected that the United States will give Iraq new assistance to combat insurgents but declined to describe it.
A senior U.S. official said Washington is considering whether to conduct drone missions for Iraq but that no decision had been made.
Tikrit, the capital of Salaheddin Province, lies roughly halfway between Baghdad and Mosul.
Turkey called for an emergency NATO meeting after ISIL militants seized 80 Turkish nationals in Mosul.
Turkey also warned of retaliation if any of its nationals, including diplomats, special forces soldiers, and children, were harmed.
The militants seized 49 people from the Turkish consulate in Mosul on June 11, one day after they abducted 31 Turkish truck drivers.
UN Secretary-General Ban Ki-moon strongly condemned the abductions and the seizure of Iraqi territory by the militants.
He urged "the international community to unite in showing solidarity with Iraq as it confronts this serious security challenge."
Prime Minister Nuri al-Maliki has called Mosul's fall to the militants a "conspiracy" and vowed to regain control of the city by force.
He also said that the military commanders who retreated "must be punished."
As the militants overran Mosul on June 10, members of the security forces reportedly removed their uniforms and joined residents in fleeing the city.
Some 500,000 people are believed to have fled Mosul, a city of 2 million residents.
The Iraqi parliament is due to meet on June 12 to debate Maliki's request for a state of emergency.
Iraqi Foreign Minister Hoshyar Zebari said Baghdad will cooperate with the Peshmerga, the large and disciplined military force of the semiautonomous Kurdish region, to drive the militants out of Mosul, but he did not provide details on possible cooperation.
ISIL fighters have been holding parts of Ramadi, the capital of western Anbar Province, and much of the nearby city of Fallujah since early January.
ISIL, which also controls the northeastern Syrian Province of Al-Raqqa, wants to establish an Islamic state in areas on the two sides of the border.