WELLINGTON (AFP) ― The United States is monitoring the possibility of terror groups trying to down aircraft by implanting explosives in the human body, Homeland Security chief Janet Napolitano said Wednesday.

Napolitano stressed that there was no evidence of an imminent threat from a so-called "body bomb," but said the prospect was one of the ever-changing attack strategies that security officials had to combat.

She cited the case of Nigerian "underwear bomber" Umar Farouk Abdulmutallab, who tried to blow up an airliner in 2009 with explosives hidden in his pants, as an example of the innovations adopted by militants.

"It is fair to say that there are a number of ways our adversaries have explored to get explosives aboard airplanes, such as the underwear bomber," she told reporters in New Zealand.

"Another, of course... is actually implanting explosives inside an individual.

"Do we have specific, credible evidence of a threat today? I would not say that we do. However, the importance is that we all lean forward and that, as people are travelling, we are sensitive to the ever-evolving threats that we see."

Underwear bomber Abdulmutallab managed to smuggle more than 76 grams of the explosive Pentaerythritol tetranitrate (PETN) on board a Detroit-bound Northwest Airlines flight from Amsterdam on Christmas Day in 2009.

But the al-Qaida plot failed when the bomb did not detonate properly, instead causing a fire as the plane began its descent to Detroit.

Passengers and crew members were able to restrain Abdulmutallab and extinguish the blaze, allowing pilots to safely land the plane.

He was handed four life sentences and an additional 50 years behind bars in February. (AFP)