A rash of thefts from commercial airline pilots' hotel rooms has led the pilots' unions and the government to warn fliers that terrorists might be watching them or trying to steal their uniforms and IDs. 

"Please be alert to the possibility that you may be the target of a surveillance operation," read a warning given to Northwest Airlines pilots by their union, the Air Line Pilots Association (ALPA). 

Transportation Department spokesman Chet Lunner said the government had asked airlines to tell their pilots to be more careful. 

The warnings came just before the July 4 holiday weekend, with the nation already on a high state of alert for post-Sept. 11 terrorist attacks. 

Both ALPA and the Allied Pilots Association (APA), which represents American Airlines pilots, warned their members that some flight crews believed they were being watched by people of Middle Eastern descent. 

The unions said the individuals had gathered in hotels and other places in Amsterdam, Frankfurt and London where crews hang out between flights, and were trying to overhear their conversations. 

An American flight attendant reported that one person had asked about the schedule of the van that drives crews to the airport, and an airline captain was stopped on the street and questioned by two strangers, the APA said. 

In addition, some pilots have reported that their hotel rooms were broken into and their uniforms and IDs stolen. The unions said they did not know how many theft cases there were. 

"From time to time, pilots' rooms are burglarized and they steal anything they can get their hands on," said ALPA spokesman John Mazor. "After Sept. 11, this has taken on a new security dimension." 

Capt. Sam Mayer, a spokesman for the APA, said the thefts have made him more careful. 

"You have to be leery in light of what's going on the world today," Mayer said. "I've been checking IDs a lot more closely, comparing the pictures to the face instead of giving it just a cursory glance and accepting the uniform." 

Mazor said the thefts offer another argument for tamper-proof identification cards for airline and airport employees. The cards would have biometric identifiers, such as fingerprints, to prevent someone other than their rightful owner from using them. 

"It wouldn't matter if you stole the uniform or the ID," Mazor said. "The biometric component would prevent it from being used fraudulently." 

The Associated Press contributed to this report.