American GIs dug into position in Arabia's searing sands yesterday while American diplomats worked the air-conditioned corridors of power, both tightening a vise, step by step, on an increasingly isolated Iraq.

And as Pentagon sources revealed that the U.S. might station as many as 250,000 ground troops in Saudi Arabia, the dangerous standoff in the desert continued to rattle nerves across the Middle East.

Israel fired off a test missile as a warning to the Iraqis. Air raid sirens wailed mysteriously in Jordan. Turkish villagers said Iraqi tanks had rumbled into position near the Turkey-Iraq border. And Iraq closed its frontiers for foreigners, heightening concerns about hundreds of Americans stranded in Iraq and Kuwait.

The tension reached as far as the hills of Tennessee and farms of Wisconsin, where local Air National Guardsmen were hurriedly called from their civilian jobs to help ferry U.S. troops and gear the 5000 miles to Saudi Arabia.

A Pentagon source, who spoke on condition of anonymity, said the United States was prepared to put "many divisions" in Saudi Arabia in the face of a continued Iraqi buildup in Kuwait.

"We have contingency plans that could result in the insertion of up to 200,000 to 250,000 ground forces before it's all done," said the source, who spoke on condition that further identification be withheld. Such moves could take up to two months, the source said.

The Pentagon earlier reported that 50,000 Iraqi combat units were moving south inside Iraq, and that if they entered Kuwait, the total invasion force would number "about 170,000." Only a day earlier, the intelligence estimates had put the Iraqi force at 100,000 men.

American leaders believe that oil kingdom could be the next target of