The State Department has instructed all personnel at the U.S. Embassy in Baghdad not to leave reinforced structures due to incoming insurgent rocket fire that has killed two American government workers this week.

In a memo sent Thursday to embassy staff and obtained by The Associated Press, the department says employees are required to wear helmets and other protective gear if they must venture outside even in the heavily fortified Green Zone and strongly advises them to sleep in blast-resistant locations instead of the less secure trailers that most occupy.

"Due to the continuing threat of indirect fire in the International Zone, all personnel are advised to remain under hard cover at all times," it says. "Personnel should only move outside of hard cover for essential reasons."

The memo was sent after a second American citizen was killed by a rocket attack in the Green Zone on Thursday. A U.S. citizen military contractor died of his wounds on Monday after being severely injured with four others in an attack.

"Essential outdoor movements should be sharply limited in duration," the memo says, adding that personal protective equipment "is mandatory for all outside movements."

"We strongly recommend personnel do not sleep in their trailers," it goes on to say, offering space inside the Saddam Hussein-era palace that is the embassy's temporary home as well as room at an as-yet uncompleted new embassy compound and a limited supply of cots.

One explosion from a rocket launched by suspected Shiite militiamen on Thursday ignited a fire in the central area of the zone that sent a massive column of thick, black smoke drifting over the Tigris River.

Military and diplomatic officials would not say what had been hit inside the Green Zone. A U.S. military statement said one civilian was killed and 14 wounded "in the vicinity" of the protected district.

The first wave of rockets this week came on Easter Sunday. The Green Zone — and areas nearby — have barely had a breather since.

On Sunday, at least 12 Iraqis were killed that day outside the Green Zone, apparently by salvos that went astray.

Earlier Thursday, suspected Shiite extremists hammered the U.S-protected Green Zone and other areas in Baghdad with rocket and mortar volleys Thursday, killing one American government worker.

It was the latest in a week of barrages against the heavily fortified area in central Baghdad.

U.S. Embassy spokeswoman Mirembe Nantongo said the American was a government employee but provided no other details pending notification of relatives.

It was the second death since frequent attacks against the Green Zone began on Sunday, when an American financial analyst was killed.

At least five rounds also struck near the Green Zone on Thursday, killing one Iraqi civilian and injuring another 14, the U.S. military said in a separate statement. Three other rounds hit U.S. military bases on the predominantly Shiite eastern side of the Tigris River, wounding four U.S. soldiers, it added.

Mortar rounds also struck two joint security stations and a checkpoint manned by U.S.-allied guards who have turned against al-Qaida in Iraq, injuring three of them.

The attacks on the Green Zone sent huge plumes of billowing smoke above the heavily fortified compound that houses the U.S. and British embassies as well as the Iraqi government.

Iraqi police said two civilians also were killed Thursday when a rocket hit a house in Sadr City, a stronghold of the Mahdi Army militia that is loyal to radical Shiite cleric Muqtada al-Sadr.

At least eight other Iraqis were killed after rounds fell elsewhere in Baghdad, police said.

The rocket and mortar attacks on the Green Zone come amid escalating tensions between Mahdi Army fighters and Iraqi security forces that threaten to unravel a cease-fire and reverse a monthslong period of calm across the country.

The U.S. military has blamed breakaway factions of al-Sadr's movement believed to be funded and trained by Iran, which has denied the allegations.

American troops came under fire after they killed two suspected militants believed to be doing surveillance for the attacks, but no U.S. casualties were reported, according to the statement.

A U.S. patrol also was struck by a roadside bomb in the eastern Shiite neighborhood of New Baghdad, wounding three American soldiers, it added.

"These rogue elements are haphazardly firing rockets and mortars, killing and injuring innocent Iraqi and governmental civilians," said Col. Allen Batschelet, the chief of staff for U.S. forces in Baghdad. "Our response will be measured, targeted and precise."

On Wednesday, 16 rockets struck the Green Zone, injuring one soldier with the U.S.-led coalition, two American civilians and an Iraqi soldier.