The cost of supporting the growing U.S. military force in the Middle East could total $30 billion for fiscal 1991, according to administration authorities.

The figure is double the estimates projected by Defense Secretary Richard B. Cheney two months ago, before President Bush's decision to nearly double the number of American troops opposing Iraqi forces in the Persian Gulf.

The new figure includes the price of transportation, fuel, supplies and operating expenses for supporting more than 400,000 U.S. troops and sailors in the gulf area for fiscal 1991, officials said.

Pentagon officials are still fine-tuning the estimates, and said that precise costs are difficult to project because of uncertainties over the length of time troops will be committed to the region and the amount of financial support to be provided by allies and Saudi Arabia.

"This assumes that there will be no shots fired in anger and that the level of force there on Jan. 15 will be there the rest of the fiscal year -- which may or may not come to pass," said one administration official. Fiscal 1991 began Oct. 1 and ends Sept. 30.

Military and congressional authorities also said they are uncertain how much money and other contributions will be made by allied forces supporting the operation. While other nations have pledged $6.5 billion in cash, equipment and services, about $4.2 billion has been paid thus far, according to Pentagon calculations.

Saudi Arabia also has agreed to cover the costs of fuel, food and local transportation used by U.S. military forces in the region, authorities said. American military contractors in Saudi Arabia said the kingdom has reimbursed the United States about $760 million since the operation began in early August.

Military officials said that one of the largest expenses of the operation is the transportation involved in air- and sea-lifting forces to the Middle East. The Air Force has been forced to charter dozens of commercial airplanes and the Navy has leased more than 100 ships to help move the troops, equipment and supplies needed for Operation Desert Shield.

In addition, the military is calculating the costs of paying up to 188,000 National Guard and reserve troops full-time salaries, another significant portion of the expenses.

While Saudi Arabia is supplying most of the fuel for trucks, aircraft and ships in the region, the United States still is grappling with increasing fuel costs for the movement of troops from the United States and Europe.

The hostile desert environment of Saudi Arabia is also taxing military equipment and weapons, requiring higher levels of maintenance and more spare parts because of increased wear and tear.

Shortages of some war reserves also has prompted the military to increase production rates of some missiles and ammunition, along with emergency purchases of chemical protection gear, desert combat fatigues and other equipment.

Pentagon officials said they hope to offset some of the $30 billion in costs with more contributions from allied nations and by shaving some of the Defense Department's internal budgets.

Cheney will likely submit the request for supplemental funding to Congress in early February, along with the administration's regular annual budget recommendations, officials said.

The Defense Department said the cost of the operation for fiscal 1990, which ended Sept. 30, totaled $2.7 billion. Congress allotted the military an extra $1.9 billion and the remainder was taken from other parts of the defense budget, officials said.