Kuwait announced plans tonight to close off the northern half of the country in preparation for a possible U.S. invasion of neighboring Iraq, with the new restrictions set to take effect Feb. 15, the day heightened security measures are implemented in the rest of the nation.

The restrictions were described as a step to protect Kuwaiti citizens living in the area. The decision to make the sandy desert south of the Iraqi border a military reservation also clears the zone for the continued buildup of U.S. ground forces and, possibly, for staging an attack on Iraq.

The chief Defense Ministry spokesman, Col. Yusuf Mulla, said Kuwait had no choice but to place the area "under armed forces' control" to help the army train for defending the country and to aid the U.S. ground forces. Senior Kuwaiti officials said in recent days that U.S. personnel here are expected to number at least 80,000 by next week.

The area will be used for "training and mass exercises, including with allied forces," said one official.

Most of northern Kuwait is a sparsely populated desert, with little civilian population. One-quarter of the country -- the north and northwest -- was already closed off by the government last fall so U.S. troops could use it exclusively.

The new order effectively doubles the amount of Kuwait's land available solely for military use. Those who live in the restricted zone will be allowed to remain in their homes, Mulla said, but will be required to register with the authorities and obtain special identification to travel from the area.

The announcement was the latest indication of war planning in this tiny emirate, which was invaded by Iraq in 1990 and has been a staunch U.S. ally since the United States led a coalition that liberated Kuwait from the Iraqis in 1991. In recent days, the Kuwaiti government has stepped up security in Kuwait City and other towns, sending 4,000 armed police and national guardsmen into the streets, setting up new checkpoints and checking documents.

More security measures are planned to take effect along with the new military zone. In the event of war, senior government officials have said in recent interviews they plan to implement their own version of martial law, allowing them to detain suspects indefinitely, censor the media and close off other areas of the country as needed.

The officials have said they fear sabotage or military retaliation by Iraq if Kuwait is used to launch a U.S.-led attack. The recent shooting by an alleged al Qaeda sympathizer of two U.S. civilian military contractors, one of whom was killed , has also raised concerns about the safety of the estimated 8,000 Americans living here.

The Kuwaiti military is not planning to participate in any offensive operation against Iraq, but will focus on protecting its borders and responding to any attacks inside the country. The military, which has undergone an extensive rearmament and rebuilding program since being destroyed during the Iraqi invasion, is also helping to guard U.S. facilities and troop movements.

Mulla said advance notice of the military restrictions was given not only because of the possible timing of a U.S. military operation but also to allow the many Kuwaiti families who are camping out in the desert during a school holiday to leave. "We don't want to ruin their vacation," he said.

Students head home from the American International School in Kuwait, which plans to close next week for six weeks as war fears grow.