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U.S. authorities say the threat from al Qaeda is as high as it's been since the 9/11 attacks, as countries around the world, including Canada, banned air cargo from Yemen after last week's foiled attack.

Transport Minister Chuck Strahl said the move is a precautionary one, as investigators attempt to track down the tentacles of a complex terror plot, which was unearthed aboard two cargo flights.

"It's just based on a general risk assessment," Strahl told reporters outside the House of Commons.

"Our partners around the world are basically following similar steps," he said. "Cargo coming from (Yemen) is a big concern."

It's feared that more bombs are currently circulating through the global shipping system, and companies have been asked to be vigilant while inspecting cargo in the days to come.

"I'm not going to rule out the possibility that there could be another piece of cargo out there," U.S. Homeland Security Secretary Janet Napolitano said.

There are no direct flights between Canada and Yemen, meaning that Yemeni cargo that arrives in another country before continuing on to Canada will have to be checked.

Canadian airline security officials say for a variety of reasons – including cost – 100 per cent of cargo is not checked.

Canada, the U.S., Germany and the United Kingdom are among the countries enacting new rules amid concern over an international mail bomb plot, which is believed to have originated in Yemen.

An intelligence officer in the United Arab Emirates told The Associated Press that an explosive-laden circuit board was providing a potential lead in the case, as serial numbers are being traced.

A cellphone circuit board and a computer printer component were used in one of the bombs last week. That device was discovered in Dubai.

Another explosive device was found on a cargo plane in England. Both packages originated in Yemen.

The plot was uncovered after Saudi Arabian intelligence officials provided a tip. The information was reportedly gleaned from an interview with a Yemeni al Qaeda militant who recently surrendered to Saudi officials.

Tribal leaders with knowledge of the investigation confirmed the reports to The AP.

U.S. Homeland Security and the FBI warned shipping companies in a nation-wide advisory that packages with extra postage and no return address should be double-checked, The AP reported.

Some shipping companies have already stopped shipments from Yemen, and Germany has cancelled all passenger flights from the country. Germany suspended cargo flights from Yemen over the weekend.

Last week, the bomb that was found aboard a plane in England had been routed through a German airport.

A German official told The AP that if the device had detonated, "the explosive effect would have been significant."

Meanwhile, Britain has banned the air cargo import of large printer cartridges as a response to the plot. The country has also brought in new restrictions on air cargo from both Yemen and Somalia.

The decision was "based on possible contact between al Qaeda in Yemen and terrorist groups in Somalia, as well as concern about airport security in Mogadishu," said British Home Secretary Theresa May.

Authorities have fingered Saudi bombmaker Ibrahim al-Asiri, 28, as their prime suspect. He is believed to be working with the Yemen-based wing of al Qaeda.

Bombs may have been 'dry run'

While the two mail bombs found last week were addressed to synagogues in Chicago, the packages may have been intended to bring down the flights themselves, U.S officials believe.

Terrorism expert Alan Bell said the plot's organizers could have used resources in the United States if their true intention was to attack targets in Chicago.

"So that leaves us with a couple of other choices," Bell told Â鶹´«Ã½ Channel Monday evening.

"One is to bring a plane down to basically cause a major problem for the global freight services, or they were using this as a dry run to see exactly what happened, what the response was from all the security services around the world, and obviously to keep their name in the media."

Bell said that Yemen is of particular concern because al Qaeda operatives have exploited its lack of strong government and military intelligence service to set up shop and use as a base for planning and implementing their attacks.

A young female computing student was arrested in the plot over the weekend, but she has since been released because officials said that someone forged her signature on a shipping manifest.

The arrest and subsequent release highlight the difficulty of tracking senior members of Al Qaeda in the Arabian Peninsula, which is the terror group's local affiliate.

The plot also exposes holes in air freight security despite more stringent regulations for passengers after the terrorist attacks of Sept. 11, 2001.

"All the money, resources and security have gone on securing commercial aircraft. Everyone who's travelled knows what you've got to go through to get on the plane," Bell said.

"And then the authorities promptly put freight in the body of the plane, which has been sanitized as far as the people who are boarding are concerned, that's not been checked."