KUWAIT CITY  —  U.N. observers ceased all operations on the Iraq-Kuwait border Monday, moving to a heightened state of alert for U.N. workers there, a U.N. spokesman said.

The roughly 800 U.N. employees still on both sides of the border were assembling in the early morning, awaiting expected word later in the day on whether to pull out entirely, said Daljeet Bagga, spokesman for the U.N. Iraq-Kuwait Observation Mission. U.N. authorities early Monday elevated their level of alert to stage 4, meaning a halt to all work. In stage 5, the final phase, they would evacuate.

"We are still in the DMZ, but we have stopped our operations ... patrolling and monitoring," Bagga said by telephone from the border. "We are still reviewing the situation with U.N. headquarters."

U.N. observers, some armed, have been monitoring the 16-mile wide zone between Iraq and Kuwait since shortly after the 1990-91 Gulf War. The 120-mile border is heavily enforced, with earthen berms, ditches, and an electrified fence. Kuwait built the fence after the war to try to stop incursions by Iraqi agents.

The U.N. mission had pulled 400-500 workers from the border last week, as the United States talked of time running out for Saddam Hussein to avert attack by nearly 300,000 allied forces massed in the Gulf.

President Bush said that Monday would be a last day for diplomacy.

Before last week's pullback, the United Nations had a total of 1,332 workers on the border, according to Bagga.

U.N. workers said employees on the Iraqi side were heading Monday to the Kuwaiti side.

U.N. monitors complained to the Security Council that they had found at least three gaps cut in the fence, and said they believed U.S. Marines were responsible.