The formal warning to Serbia is expected to come from leaders of the Group of Seven - the United States, Germany, Canada, France, Britain, Italy and Japan - tomorrow, the third and last day of the group's 18th annual economic summit here.
Brent Scowcroft, President Bush's national security adviser, said any U.S. military role would likely be confined to providing air and naval protection for relief efforts. He indicated that troops from other nations would probably accompany truck convoys carrying food and supplies if military protection is necessary.
Bush administration officials left open the possibility that U.S. ground combat troops could be part of a military mission to the region, but said such a role was unlikely. Earlier, administration officials had all but ruled out the use of U.S. ground troops.
"All options are open. We don't rule anything out," said White House spokesman Marlin Fitzwater. But a senior administration official said, "I think that U.S. ground combat forces are a real long shot."
In Washington, a Navy official said the Pentagon's most powerful punch in the region, the carrier Saratoga and its 80 warplanes, was on "routine maneuvers" in the Mediterranean and not headed toward the former Yugoslavian republics. Hundreds of Air Force planes could also be dispatched to bases in Italy for action, defense officials said.
Last week, Defense Secretary Dick Cheney told reporters that Air Force and Navy warplanes might be used to escort ground convoys from Adriatic ports to Sarajevo, the capital of Bosnia, about 125 miles inland.
Cheney said some U.S. ground forces, including air traffic controllers and supply experts, might be deployed to the city to help in the effort.
Defense officials have said U.S. air forces could move out within 48 hours of a U.N. request for help.
Aides to the leaders at yesterday's meeting said they expected the airlift to be supplemented with ground convoys carrying relief supplies from the port of Split in neighboring Croatia.
Meanwhile, in Washington, the Treasury Department expanded its economic embargo of Yugoslavia to include all companies based in Serbia and Montenegro.
On June 1, the department had ordered U.S. businesses and individuals not to do business with Yugoslavian government-owned businesses. At the same time, it froze all U.S. assets of Yugoslavian government businesses.
An international airlift began last week to provide food and supplies to the people trapped in Sarajevo. Serbian forces withdrew from the airport to permit U.N. peace-keeping troops to take control.
A senior Bush administration official, briefing reporters here, described the airlift operation as "fragile" and said that even if the relief effort continues, it cannot begin to supply enough food for Sarajevo and other cities in Bosnia.
"We also need to get relief to these other places in Bosnia, and . . . that will probably be done predominantly if not exclusively by road convoy," the official said. "So there's another big humanitarian relief effort out there."
In addition to discussing options against Serbia, the summit participants conferred on economic and trade issues but did not appear to be on the verge of any major actions.
U.S. Treasury Secretary Nicholas F. Brady said it was highly unlikely that any breakthroughs would occur on the stalled global trade talks while the world leaders are here.
"Is it going to happen in the next three days? I don't think so," Brady said. "Is it going to happen by the end of the year? Maybe. Is it going to happen within the next year or so? Absolutely. It has to."