Canada has banned all air cargo originating from Yemen in the wake of last week's discovery of explosive material on flights bound for the United States.

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Saudi fugitive Ibrahim Hassan al-Asiri, shown in this combination picture taken from a Yemeni police handbook of most-wanted terror suspects, is the main suspect in the Yemeni bomb plot. ((Reuters))

The ban was not instituted based on intelligence specific to Canada, said Transport Minister Chuck Strahl. It is instead an interim measure to keep people safe while the government decides what further steps might be necessary, he said.

Earlier Monday, Germany suspended all passenger flights from Yemen until further notice. It had previously banned all cargo flights from the country after it was determined that one of the packages was routed to London through the UPS hub in Cologne.

Britain has banned all unaccompanied freight packages from Yemen and Somalia, and barred all passengers from transporting toner cartridges larger than 500 grams in their carry-on luggage as of Monday night.

Air security tightened

Officials on both sides of the Atlantic say they're reviewing air cargo security. In many countries, including Canada, air cargo is often not screened as rigorously as passengers and luggage.

"If they can send explosives through air cargo, they can do a tremendous amount of damage to planes, material and even to people," said Peter St. John, an aviation security expert in Winnipeg.

One of the two bombs mailed from Yemen did travel on a passenger plane. The Canadian government says about 75 per cent of commercial cargo in the country is transported in aircrafts that also transport passengers.

Earlier Monday, a team of U.S. investigators headed to Yemen to help search for suspects in the mail bomb plot.

British Prime Minister David Cameron says the powerful bombs, which contained industrial-grade explosives and were hidden in packed computer printers, may have been aimed at bringing down the planes.

The Yemen-based group al-Qaeda in the Arabian Peninsula has been linked to the bomb plot because of the explosive pentaerythritol tetranitrate, or PETN, which was used by the group in the bombing attempt last Christmas Day of a Detroit-bound airliner.

The two devices contained 300 grams and 400 grams, respectively, of the highly explosive material, enough for a "significant" explosion, a German official said Monday.

U.S. authorities also had intelligence that Yemeni al-Qaeda was planning this operation, according to a U.S. official who spoke on condition of anonymity to discuss matters of intelligence.

Two suspects sought

There are two main suspects in last week's incident.

The first is Saudi-born Ibrahim Hassan al-Asiri, a bomb-making expert for al-Qaeda in the Arabian Peninsula, who is wanted for making the device in the attempted Christmas Day bombing.

London's the Sun newspaper reports the other main suspect is Anwar al-Awlaki, a U.S.-born member of the same al-Qaeda group.

The tip that led to thwarting the bomb plot came from a leading al-Qaeda militant in Yemen who surrendered to Saudi Arabia last month, officials said Monday.

Officials said Jabir al-Fayfi, a Saudi militant who had joined al-Qaeda in Yemen, told Saudi officials about the plan. The officials spoke on condition of anonymity because they were not authorized to talk to the media.

With files from The Associated Press