U.S. officials and NATO leaders forged ahead with plans to rescue about 200 U.N. peacekeepers held hostage by Serbs in Bosnia, even though U.S. officials are uneasy about President Clinton's offer to send in up to 25,000 U.S. troops to complete the mission. Defense Secretary William Perry said the U.N. would have to make the decision to move in -- action that, ironically, would signal a larger U.S. intention to stay out of the war. But if it happens, he added, the American soldiers would be prepared to handle Serbs with "overwhelming force." (The Bosnian Serbs have said they're holding the peacekeepers as insurance against further NATO air strikes.) But even as NATO itself gave the go-ahead for the evacuation, Western officials maintained they had not decided to support a full U.N. withdrawal from the region. "There hasn't been a decision to withdraw," said a U.S. official. "This is only contingency planning at this point."