The White House on Wednesday expressed concerns that Islamic militants had regained a foothold in Iraq after an al Qaeda-affiliated group seized control of a second major city.
Islamist militants seized the northern city of Tikrit on Wednesday, an action that sparked alarm in Washington and Baghdad, just days after rebel forces also captured Mosul, Iraq's second largest city.
"There is no doubt that the situation has deteriorated over the last 24 hours," he added.
Earnest said the U.S. was "deeply concerned" the instability could create a humanitarian crisis, with reports saying Iraqi security forces had fled both cities and thousands of refugees were seeking shelter.
Rebel groups have allegedly seized control of government buildings and released prisoners, adding to the chaos.
Earnest called on Iraqi leaders to organize a response to turn back the rebel forces, and said Washington was offering its support to Baghdad.
The Iraqi government plans to meet Thursday in Baghdad to vote on whether to give Iraqi Prime Minister Nouri al-Maliki emergency powers that would give him broader latitude to combat the sectarian violence.
The U.S. also condemned the kidnapping of 49 Turkish diplomatic personnel in Mosul by the group the Islamic State of Iraq and the Levant. Earnest called the attack "despicable" and demanded the immediate release of the prisoners.
State Department spokeswoman Jen Psaki said earlier Wednesday that Secretary of State John KerryJohn KerryCongress, Trump need a united front to face down Iran One year ago today we declared ISIS atrocities as genocide Trump’s realism toward Iran is stabilizing force for Middle East MORE had called Turkish Foreign Minister Ahmet Davutoğlu to express "their mutual concern" about the security situation.
"We join Turkey and the international community in calling for the immediate release of Turkey's kidnapped diplomatic personnel," Psaki said.