"We are relieved to learn the prime minister was unharmed. This apparent act of terrorism, which we strongly condemn, was directed at the heart of the Iraqi state," State Department spokesman Ned Price said.

"We are in close touch with the Iraqi security forces charged with upholding Iraq's sovereignty and independence and have offered our assistance as they investigate this attack," he added.

Khademi called for calm after the attack on his residence in Baghdad's Green Zone, saying he was unhurt.

His office characterized the attack as a "failed assassination attempt."

Two security sources earlier confirmed the attack in the heavily guarded Green Zone, which also hosts the US embassy and is frequently targeted by rocket attacks.

A large number of security forces were deployed in and around the Zone following the attack, according to a security source.

The attack came amid soaring political tensions over the results of October 10 elections.

Preliminary results of that poll saw the Conquest (Fatah) Alliance, the political arm of the pro-Iran multi-party Hashed al-Shaabi paramilitary network, suffer a substantial decline in its parliamentary seats. The group's supporters have denounced the outcome as "fraud."