BAGHDAD - A sharp increase in mortar attacks on the Green Zone - the onetime oasis of security in Iraq's turbulent capital - has prompted the U.S. Embassy to issue a strict new order telling all employees to wear flak vests and helmets while in unprotected buildings or whenever they are outside.

The order, obtained by the Associated Press, has created a siege mentality among U.S. staff inside the Green Zone after a recent suicide attack on parliament. It has also led to new fears about long-term safety in the place where the United States is building a huge, expensive embassy.

The situation marks a sharp turnaround for the heavily guarded Green Zone - long viewed as the safest corner of Baghdad with its shops, restaurants, American fast-food outlets, and key Iraqi and American government offices.

The security deterioration also holds dire implications for the Iraqi government, which uses the Green Zone as a haven for key meetings crucial to its ability to govern.

Reporters covering Vice President Cheney's visit yesterday were hustled into a secure area when a large explosion rattled windows in the U.S. Embassy late in the afternoon. Cheney spokeswoman Lea Anne McBride said the vice president's meeting "was not disturbed and he was not moved."

The increase in mortar attacks comes despite the presence of tens of thousands more American and Iraqi soldiers in the streets of Baghdad for the security crackdown ordered by President Bush in January.

The vest and helmet security order was issued May 3, one day after four Asian contract workers working for the U.S. government were killed when rockets or mortars slammed into the Green Zone.

It was at least the third straight day of barrages against the 3.5-square-mile area along the west bank of the Tigris River in the center of Baghdad.

Because of the "recent increase of indirect fire attacks" - the military term for mortar and artillery barrages - the order told embassy employees that until further notice, "outdoor movement" must be "restricted to a minimum."

"Remain within a hardened structure to the maximum extent possible and strictly avoid congregating outdoors," the order said.

Government employees who work outside of a "hardened structure" 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.

Attacks on the Green Zone have occurred from time to time since the first months of the U.S. presence in Iraq. Often, the rounds landed in open fields - part of a system of parks that Saddam Hussein built when the area served as the headquarters of his regime.

But the latest attacks have been unnerving because of their frequency, the size of the ordnance and their accuracy.

Some rounds appear to have been fired from Sunni insurgent strongholds to the south of the Green Zone. Others have come from areas where Shiite militiamen operate.

At last week's regional summit in the Egyptian resort of Sharm el-Sheik, U.S. Ambassador Ryan Crocker said it was unclear if the attackers were becoming more skilled, had better weapons or tools or were just getting lucky.

He noted that it was difficult to stop mortar attacks. Extremists can carry the weapons in vehicles, set up quickly, fire them and drive away.