LONDON (Reuters) - Governments tightened aviation security on Monday after two U.S-bound bombs sent in air cargo from Yemen were intercepted in Dubai and Britain.

The devices, discovered on Friday, were hidden in printers and would have been powerful enough to destroy the planes carrying them, Britain said.

The plot highlighted what appeared to be a loophole in air cargo security after Qatar Airways confirmed the Dubai parcel had been transported on its passenger planes from the Yemeni capital Sanaa via Doha.

Britain said it believed the attempt was organized by al Qaeda in the Arabian Peninsula (AQAP), but said it had no information the organization was planning further attacks.

A U.S. official has said Saudi bomb maker Ibrahim Hassan al-Asiri, believed to be working with AQAP, is a key suspect.

As a precaution Britain said it was banning air passengers from taking large toner cartridges onto planes as hand luggage, while Nigeria said it would improve the scanning of cargo bound for the United States.

Airline security in Africa’s most populous country has come under heightened scrutiny after a failed Christmas Day bombing attempt on a U.S. airliner blamed on a Nigerian passenger with explosives concealed in his underwear and claimed by AQAP.

The Dutch anti-terrorism agency banned all airborne post and freight from Yemen entering the Netherlands. Germany said it had suspended passenger flights from Yemen, and was considering expanding a cargo flight ban to other unnamed countries.

Britain said it was also banning all air freight sent from Somalia, adding to a ban on Yemen cargo flights imposed at the weekend.

EXPLOSIVES

British Prime Minister David Cameron thanked the police and intelligence operation whose efforts “clearly prevented the terrorists killing and maiming many innocent people whether here or elsewhere in the world.”

One of the packages was found on a United Parcel Service cargo plane at East Midlands Airport, north of London, on Friday. The other bomb was discovered in a parcel at a FedEx facility in Dubai.

The bomb found in Britain was hidden in a Hewlett Packard printer and contained 400 grams of the highly potent explosive pentaerythritol trinitrate (PETN), with the Dubai package holding 300 grams, a German government source said. The Christmas Day attack also used PETN.

British authorities intervened after a tip-off from Saudi intelligence was passed on by German authorities, the German source added.

The BBC, citing unidentified British officials, said the information came from an al Qaeda member who turned himself into Saudi authorities.

The plot could fuel calls for the wider use of imaging technology designed to detect explosives, which is not standard, but freight firms are reluctant to bear the full cost.

Tighter international air cargo security rules could deal a blow to trade and the world economy as it recovers from the global recession. According to airlines association IATA, about 35 percent of the value of world trade is carried by air.

Freight firms clashed with U.S. and European policy makers last year over calls for 100 percent scanning of sea containers. Plans to introduce full scanning from 2012 were postponed.

Airlines could also face pressure to put less cargo on passenger planes or improve blast protection.

An Israeli security expert said it would have been “nearly impossible” to spot the Yemeni bombs using standard X-ray machines.

“The structure of a printer is so dense that it is very difficult to spot explosives or a detonator through visual screening,” said Yuval Amsterdam of Tamar Explosive Simulants Technologies, which produces dummy bombs for security drills.

Handout image shows a toner cartridge discovered aboard a cargo plane in East Midlands, north of London, October 29, 2010, which U.K. officials discovered had been manipulated with wires attached and white powder. REUTERS/CBS News/Handout

FREIGHT SECURITY

Philip Baum, editor of Aviation Security International magazine, said new technology would not eliminate the risk or airline attacks.

“The only thing that has prevented things has either been good luck or people,” he told Reuters.

Interior minister Theresa May said Britain would review all aspects of air freight security.

“At this stage we have no information to suggest that another attack of a similar nature by al Qaeda in the Arabian Peninsula is imminent,” May told parliament.

“But this organization is very active. It continues to plan other attacks in the region, notably against Saudi Arabia.

“We therefore work on the assumption that this organization will wish to continue to find ways of also attacking targets further afield.”

Britain’s toner ban adds to the list of items passengers are already barred from carrying on board.

Restrictions on the carrying of liquids on planes were introduced in 2006 after the discovery of an al Qaeda-inspired plot to blow up transatlantic flights with explosive materials concealed as soft drinks.

In Sanaa, cheering Yemenis greeted the student detained briefly on suspicion of having sent the bombs.

Yemeni police arrested computer science student Hanan al-Samawi on Friday after tracing her through a telephone number left with a freight company but released her the next day, saying she had been a victim of identity theft.

A U.S. technical team has arrived in Yemen to train Yemenis in the use of sophisticated airport screening equipment, a government official told Reuters.