The US has warned airlines of a potential threat from militants surgically implanting explosives.
It came after the Department of Homeland Security issued a memo to security officials and foreign counterparts alerting them to the threat of "body packing".
No specific plot has been identified but an anonymous official said new intelligence highlighted the threat.
Air passengers could now face even tougher screening measures.
In a statement, the Transportation Security Administration (TSA) said that due to "significant advances in global aviation security in recent years, terrorist groups have repeatedly and publicly indicated interest in pursuing ways to further conceal explosives.
"As a precaution, passengers flying from international locations to US destinations may notice additional security measures in place.
"These measures are designed to be unpredictable, so passengers should not expect to see the same activity at every international airport.
"Measures may include interaction with passengers, in addition to the use of other screening methods such as pat-downs and the use of enhanced tools and technologies."
Existing airport screening methods cannot detect plastic explosives under the skin, TSA spokesman Nicholas Kimball told the Los Angeles Times.
The memo sent to security officials was obtained by the Associated Press, which quoted it as saying that "body packing" was a "criminal tactic with possible terrorist application".
Some militant organisations seeking to mount an attack on aviation have used increasingly creative methods in their efforts to evade detection in recent years.
Al-Qaeda in the Arabian Peninsula (AQAP) has been linked to the attempt by a Nigerian man to bring down a plane over Detroit using a bomb concealed in his underpants in 2009.
The group was also thought to be behind an attempt in 2010 to bomb cargo planes using printer cartridges sent by post.
But experts are divided over the feasibility of mounting an attack using implanted explosives.
Chris Ronay, a former chief of the FBI explosives unit, told AP it would be "rather easy" once a willing would-be suicide bomber was found, the explosives secured and the bomb made.
Dr John Clifford Jones, an explosives expert at the University of Aberdeen, said the quantity of explosives that could be surgically implanted would not be enough to blow up a plane but might be enough to break open the fuselage.
"Energy to initiate the action of the explosive, making it detonate," he told the BBC, "could be provided by something incorporating a 9-volt battery," which the bomber might have to carry along with them, disguised as something like a mobile phone.
"Yet my intuition is that, all things considered, concealment followed by 'successful' operation of the device seems quite implausible."