Earlier this year, a missile fired by a CIA-operated drone killed a Yemeni doctor who had devised medical procedures which could be used to surgically plant explosive devices in humans, several U.S. officials told Reuters.

However, another individual, the expert bomb-maker who came up with this tactic survived a similar missile attack last year. Counterterrorism agencies believe he is still engaged in active plotting against U.S. and other Western targets.

Moreover, three U.S. officials said counter-terrorism agencies report that other doctors in Yemen are prepared to surgically load bombs into the organs of militants.

The possibility of implanted bombs has been a concern for U.S. officials since at least 2009, when two incidents occurred involving militants who had spent time with leading figures of Yemen-based Al Qaeda in the Arabian Peninsula AQAP.L.

In August 2009, a Saudi militant who had spent time in Yemen unsuccessfully tried to assassinate Prince Mohammed bin Nayef, Saudi Arabia's counter-terrorism chief, with what authorities initially believed was a bomb secreted in his anal cavity.

Authorities determined the bomb was virtually identical to a one which Umar Farouk Abdulmutallab, a Nigerian militant who had been studying Arabic in Yemen, used to try to blow up a Detroit-bound airliner on Christmas Day 2009.

Both Abdulmutallab's bomb and the bomb used in the failed attack on Nayef turned out to have been sewn into the would-be bombers' underwear, rather than implanted inside body organs or cavities.

These incidents sparked concern among U.S. and western counter-terrorism agencies that implanted bombs might be a more effective way for militants to evade airport security devices including X-ray machines and metal detectors.

MORE OR LESS POWERFUL?

After the failed attack on Prince Nayef, three U.S. agencies examined the threats which bombs secreted in clothing or inside the body could pose to aviation security.

According to U.S. officials, the research suggested that a bomb hidden inside a body cavity or organ would be less likely to jeopardize the safety of an airplane than a bomb hidden under clothing. Much of the force of the bomb would be absorbed by exploding body tissue, likely killing the bomber but causing little structural damage to an aircraft.

By contrast, the explosive force of a bomb hidden under clothing alone would be more likely to cause potentially catastrophic damage to an airplane if detonated in flight, officials said.

Officials said that in response to the possible deployment of implanted bombs, efforts were being made to adjust airport security, including the body scanners and metal detectors now used, to try to spot potential threats.

AL-ASIRI CONNECTION

Officials said that one reason for concern is the continuing role of Saudi bomb-maker Ibrahim Hassan al-Asiri, now regarded by U.S. and European authorities as one of the more dangerous and imaginative AQAP operatives presently at large.

Counterterrorism experts attribute the original invention of the two tactics to Asiri, and it was his brother who wore the underwear bomb and died while attempting to kill Prince Nayef.

Officials acknowledge initial reports were wrong that Asiri was killed in the same sequence of drone attacks which resulted in the death last year of Anwar al-Awlaki, a U.S.-born militant Yemeni preacher.

ABC News reported on Monday that American and European officials feared al Qaeda may soon try to attack U.S.-bound aircraft using explosives implanted in the bodies of militants. ABC reported that due to this concern, security had been stepped up at some British and European airports and some Federal Air Marshals may have been redeployed.

(Reporting by Mark Hosenball; Editing by Jackie Frank)

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