ISTANBUL, Turkey -- The Clinton administration is considering an easing of sanctions against Serbia if the Moslem-led government in Bosnia rejects a plan for ending the war in the former Yugoslav republic, a senior U.S. official said Thursday.Secretary of State Warren Christopher is discussing that option with West European foreign ministers at a NATO meeting here. It already has the approval of France, which has threatened to withdraw its peacekeepers if there is no negotiated settlement.Spurred by a cease-fire agreement between Moslems and Serbs the administration is preparing with European allies and Russia a "reasonable solution" to the bloody dispute in Bosnia.The map, which should be ready within weeks, contains territorial and other concessions to the Moslems. As a result, it probably will get their approval, said the senior U.S. official, speaking on condition of anonymity. However, he said, "In the highly unlikely event the government rejects it we will consider lifting some sanctions against the Serbs."President Bill Clinton initially favored permitting weapons shipments to the Bosnian government but abandoned the idea when most allies objected to it.This strategy was discussed with French leaders in Paris during Clinton's visit there earlier in the week. It reflects a significant shift in that the United States, having tried to protect the Moslems against European pressure, now is willing to consider an easing of trade sanctions against the Serbs."We have to consider what we can do to get the parties to accept a settlement," the senior official said.Christopher discussed the strategy Thursday with British Foreign Secretary Douglas Hurd and German Foreign Minister Klaus Kinkel. Their governments along with those of France and Russia are members of a so-called "contact group" formed in April to seek a negotiated settlement.The plan would provide for the return of Moslems to areas in the west and east now controlled by the Serbs, and the return of Croats to their former homes in the north, also now held by Serb forces.