BAGHDAD (AP) -- Warning sirens wail and within seconds rockets and mortars strike - sometimes one or two, other times 10 or more.
The Green Zone is again a prime target as American and British diplomats, Iraqi politicians, contractors and others struggle to go about their business - always aware that any time they are outside the most fortified buildings there is a chance to be injured or killed.
The danger has temporarily reshaped life: Green Zone traffic is minimal, few people venture out on the streets and security precautions - always high - have been boosted. Many diplomats and others prefer to bunk on cots in the stone and marble grandeur of the former Saddam Hussein palace that now holds U.S. Embassy offices.
On Thursday, the State Department instructed all Embassy personnel not to leave reinforced structures. A memo sent to embassy staff and obtained by The Associated Press says employees are required to wear helmets and other protective gear if they must venture outside and strongly advises them to sleep in blast-resistant locations instead of trailers.
For the fourth day this week, suspected Shiite militiamen sent rockets and mortars into the Green Zone in central Baghdad. The volleys on Thursday began in the morning and came in about once an hour well into nightfall.
The attacks on the Green Zone are being carried out in tandem with growing clashes between Iraqi government forces and the Mahdi Army militia led by anti-American cleric Muqtada al-Sadr.
By bombarding the Green Zone, the followers of al-Sadr are not only targeting the Iraqi government, but also the hub of the American political mission and its influence on the Iraqi government.
At least one death was reported inside the Green Zone in the latest attacks. Embassy spokeswoman Mirembe Nantongo said a U.S. government employee was killed, but would give no further details until relatives are notified. Another American, a financial analyst who audited contracts in Iraq, was killed Sunday in the zone, the embassy and relatives said earlier this week.
One explosion 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.
U.S. military officials said Thursday that among the weapons used in recent attacks on the Green Zone were 107mm rockets made in Iran. One official, speaking on condition of anonymity because of the sensitivity of the information, said they have included rockets stamped with 2007 Iranian manufacture dates.
Washington has accused Iran of funneling weapons to Shiite militia. Iran denies it.
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.
A U.S. official, speaking on condition of anonymity because of security concerns, said one round earlier this week hit a main helicopter landing zone used by the U.S. forces, putting it temporarily out of commission. Housing used by some U.S. officials and contractors was also hit, the official said.
"All personnel are required to wear body armor, helmet and protective eye wear any time they are outside of building structures in the International Zone," said embassy spokeswoman Nantongo, using the official name of the area. "Beyond that, we don't discuss our security posture."
Another U.S. official said that personnel - who usually sleep two to a trailer on the embassy grounds - are now sleeping inside the former Saddam palace where their offices are located.
"There are cots everywhere," the U.S. official said. "People are scouting out free couches."
The official - who has been through other attacks - described the recent barrages as "qualitatively different."
"There is a sense of hunkering down for a sustained period of time," the official said, speaking on condition of anonymity because of security restrictions.
The Green Zone is normally a five-square-mile haven from the war, particularly in recent months as overall violence in Baghdad has dropped. But it is now caught in the widening attacks by forces loyal to al-Sadr to protest a crackdown by the Iraqi government.
The last sustained attacks on the Green Zone were in July when extremists unleashed a barrage of more than a dozen mortars or rockets, killing at least three people - including an American - and wounding 18.
Security forces know where the firing is from: mainly Shiite districts in eastern Baghdad. The challenge is how to stop them.
The attacks come from deep inside residential neighborhoods, making it difficult to counterattack without risking widespread civilian casualties. Also, the rockets and mortars are fired from mobile launchers, meaning militants can speedily move away from the launch area before soldiers can respond.
Last May, an explosion from a rocket rattled windows in the U.S. Embassy while Vice President Dick Cheney was visiting. In the same month, a blast hit the British Embassy compound when then Prime Minister Tony Blair arrived.
In March, a rocket or mortar round landed near Iraqi Prime Minister Nouri al-Maliki's office while he and U.N. Secretary-General Ban Ki-moon were holding a news conference. Ban ducked behind a podium as small chips of debris floated down from the ceiling.
Attacks against the zone have also renewed concern about security at the new U.S. Embassy, which is due to open this year within a protected area along the Tigris. The embassy will be Washington's largest and most expensive foreign mission.