The U.S. Embassy has ordered its staff to wear flak jackets and helmets while outdoors or in unprotected buildings following an increase in mortar and rocket attacks against the heavily protected Green Zone.

The order, obtained Wednesday by The Associated Press, was issued last week after four Asian contract workers were killed during a barrage into the Green Zone, a 3.5-square mile area along the west bank of the Tigris River in the center of Baghdad. The area contains the U.S. and British embassies and many key Iraqi government offices.

U.S. government employees who work outside of a "hardened structure" such as the current embassy building or travel "a substantial distance outdoors" must wear "personal protective equipment," meaning flak jackets and helmets, the order said.

A U.S. Embassy spokesman confirmed the order was in effect until further notice. But he refused to say more, citing security, and would not allow his name to be published, citing embassy regulations.

Mortar and rocket attacks have occurred from time to time since the early months of the U.S. presence in Iraq. But the recent attacks have raised new concern since they are occurring despite the U.S.-led crackdown, which has put thousands more American soldiers on the streets in attempts to restore order.

It's not clear what groups have been responsible for the recent attacks on the Green Zone. Some barrages have been launched from Shiite-dominated areas, but the Green Zone is also within range of Sunni militant strongholds.