Calling Bosnian Serbs "outcasts and international pariahs" forshooting down an American F-16 today, President Clinton then spent much of the day closeted with his national security advisors considering what action to take next. NATO officials have launched a massive search-and-rescue mission to locate the pilot, who was downed over northern Bosnia. The plane was flying at approximately 20,000 feet on an air patrol mission enforcing the no-fly zone overBosnian Serb territorywhen it was hit by a Russian SA-6 surface-to-air missile. Only the pilot was aboard. While it is not yet known whether he survived the crash, NATO sources report unconfirmed information that Bosnian Serbs have taken the pilot hostage.TIME Pentagon correspondent Mark Thompsonsays that any American action is likely to be low-key, focused on trying to recover the pilot safely if he survived the crash. Writes Thompson: "The problem is, we've got ourselves into this situation where we can't do anything without the consent of our NATO allies, who have all these people of their own on the ground. In addition, Clinton does not seem prepared to send in ground troops."