US officials, meanwhile, said there are indications that Syria launched airstrikes into western Iraq yesterday in an attempt to slow the al-Qaeda-inspired insurgency fighting both the Syrian and Iraqi governments.
Officials said the strikes appeared to be the work of Syrian President Bashar Assad's government, which is locked in a bloody civil war with opposition groups.
The target of the attacks was the extremist group Islamic State of Iraq and the Levant, which has been fighting along with the rebels opposed to Assad and seeks to carve out a purist Islamic enclave across both sides of the Syria-Iraq border.
"The solution to the threat confronting Iraq is not the intervention of the Assad regime, which allowed ISIL to thrive in the first place," said Bernadette Meehan, a National Security Council spokeswoman.
"The solution to Iraq's security challenge does not involve militias or the murderous Assad regime, but the strengthening of the Iraqi security forces to combat threats."
US officials believe the leadership in Baghdad should seek to draw Sunni support away from the militants led by the Islamic State.
Several politicians, including Ayad Allawi, a secular Shiite who has been named as a possible contender to replace al-Maliki, have called on him to step down and form an interim government that could provide leadership until a more permanent solution can be found.
Al-Maliki, however, insisted the political process must be allowed to proceed following recent national elections in which his bloc won the largest share of parliament seats.
"The call to form a national salvation government represents a coup against the constitution and the political process," he said.
He called on "political forces" to close ranks in the face of the growing threat by insurgents, but took no concrete steps to meet US demands for greater inclusion of minority Sunnis.