No matter how an evacuation might proceed, he said, "It cannot be done without American support."
Most of the U.S. help would likely be air support. Although about 2,000 U.S. Marines have been dispatched to the region for possible help in an evacuation, it is not clear what their role would be, because U.S. officials have said that the troops will not set foot on Bosnian soil.
As for what becomes of the cities and towns and enclaves they leave behind, the officials generally conceded that they would be at mercy of surrounding Serbian forces, with no assurance of any relief supplies to help them hold out.
Any decision to withdraw would be up to the U.N. Security Council, and it is not scheduled to take up the matter until Jan. 10, when the United Nations' mandate in the former Yugoslavia comes up for renewal.
British and French officials have offered assurances that their soldiers, at least for now, will not withdraw unilaterally, although British Lt. Gen. Sir Michael Rose, commander of U.N. forces in Bosnia, openly questioned the use of remaining longer if the war spins further out of control.
Such sentiments have been reinforced by a recent wave of hostage taking. Serbian soldiers have kidnapped or surrounded more than 400 U.N. troops and military observers.
Gilbert Lewthwaite contributed to this article from The Sun's Washington Bureau.