A mortar attack on a building housing offices of the U.S. Embassy in Baghdad Thursday wounded one American, an embassy spokesman said.

The mortar round hit the roof of one of Saddam Hussein's former palaces inside Baghdad's heavily fortified Green Zone (search), a spokesman for the U.S. Embassy said on condition of anonymity.

A gray plume of smoke could be seen rising from the multistoried building, topped by a green dome, after the blast. Dozens of U.S. diplomats work in the building alongside officials of the U.S.-led military coalition.

U.S. Ambassador John Negroponte (search), who works out of the nearby chancellery, was not in Baghdad at the time, the spokesman said.

The U.S. military said in statement earlier that two mortar rounds exploded in the Green Zone, injuring two people. The statement gave no other details and it was not clear if it included the mortar attack on the embassy building.

Thousands of American and Iraqi citizens work in the riverside enclave, which is a frequent target of guerrilla attacks.

The building hit Thursday used to serve as the headquarters of the former U.S. occupation authority.

"One member of the embassy staff was close enough to be shaken, his ears were ringing, but he didn't require medical treatment," the official said. "The other was injured and required some medical attention, but nothing serious."