LONDON - THE mail bombs discovered aboard cargo jets in England and Dubai could very easily have ended up on passenger planes, which carry more than half of the international air cargo coming into the US, experts say.

And experts caution that cargo, even when loaded onto passenger planes, is sometimes lightly inspected or even completely unexamined, particularly when it comes from countries without well-developed aviation security systems.

About 60 per cent of all cargo flown into the US is on passenger planes, according to Brandon Fried, a cargo security expert and executive director of the Airforwarders Association. New jumbo jets flying in from overseas - like the Boeing 777 - have 'cavernous' bellies where freight is stored, he said.

Most countries require parcels placed on passenger flights by international shipping companies to go through at least one security check. Methods include hand checks, sniffer dogs, X-ray machines and high-tech devices that can find traces of explosives on paper or cloth swabs.

But air shipping is governed by a patchwork of inconsistent controls that make packages a potential threat even to passenger jets, experts said on Saturday. Security protocols vary widely around the world, whether they're related to passenger aircraft or cargo planes.

That at least two parcels containing explosives could be placed on cargo-only flights to England and Dubai, one in a FedEx shipment from Yemen, was a dramatic example of the risks, but the dangers have been obvious for years, analysts said.

Passenger planes at risk too