One of its engine exploded, but it managed to land safely, officials said.

"It looks like its number four engine was hit by a surface-to-air missile, but it was able to turn around, come back and land," the senior official said, speaking on condition of anonymity.

The air force's Air Mobility Command, which sent a team of experts from Europe to investigate, said the giant cargo plane's engine exploded.

"Initial reports indicate the incident is the result of hostile action from the ground, but the type of weapon and other details are unknown," the command said in a statement.

Earlier, the air force said the C-5 declared an inflight emergency "because of excessive engine vibrations in their number four engine."

"The aircraft had just departed the airport when the problem occurred. The crew was able to land safely. There were 63 passengers and crew members on board the aircraft. No injuries were reported," it said.

No one aboard the aircraft -- 11 crewmembers and 52 US military personnel -- were injured, the air force said.

The aircraft was assigned to the 22nd Airlift Squadron at Travis Air Force Base in California.

It would be the third time since May 1, when major combat operations were declared over, that a plane has been hit by a surface-to-air flying out of Baghdad international airport.

On December 10, a defense official in Washington said an Air Force C-17 cargo and troop transport plane was hit by a surface-to-air missile after takeoff from Baghdad with a crew of three and 13 passengers.

On November 22, a DHL cargo plane was hit by a shoulder-fired SA-14 surface-to-air missile as it took off from Baghdad airport. DHL temporarily suspended flights into Iraq after the incident.

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