Insurgents penetrated Baghdad's heavily fortified Green Zone and detonated explosives at a market and a popular cafe Thursday, killing 10 people, including four American civilians, in the first bombings inside the compound housing the U.S. and Iraqi government headquarters.

A top Iraqi official said the attacks appeared to have been suicide bombings. The attack was a bold assault on the heart of the U.S.-Iraqi leadership of the country and a district seen as one of the few relatively safe refuges for Americans in the capital.

Witnesses said two men, each carrying a backpack but not required ID badges, entered the Green Zone Cafe full of Americans and other patrons at around lunchtime, drank tea and talked to each other for nearly half an hour, one of them appearing to reassure his more nervous colleague.

One of them then left and soon after an explosion was heard, then the man who remained in the cafe detonated his bomb moments later, ripping through the building, said an Iraqi vendor who was in the cafe at the time. A bomb was found and defused at the same cafe last week.

Tawhid and Jihad, the militant group of Jordanian terror mastermind Abu Musab al-Zarqawi claimed responsibility for the blasts, saying they were "martyrdom" or suicide attacks.