U.S. intelligence officials are concerned that al-Qaeda is trying to develop a new and improved bomb that could go undetected through airport security.

There is no indication that such a bomb has been created or that there's a specific threat to the U.S., but the Obama administration on Wednesday called for tighter security measures at foreign airports that have direct flights to the U.S.

American intelligence has picked up indications that bomb makers from Yemen-based al-Qaeda in the Arabian Peninsula have travelled to Syria to link up with the al-Qaeda affiliate there, known as the Nusra Front, according to a counterterrorism official who spoke on condition of anonymity because he was not authorized to speak publicly about the matter by name. The enhanced security measures have been in the works for the past month, he said.

Transport Canada said it was taking "additional precautionary measures" and would continue to assess the situation.

British airports also stepped up security after the reports.

"This is something that we've discussed with the Americans and what we've done is put in place some extra precautions and extra checks," Prime Minister David Cameron told reporters.

"The safety of the travelling public must come first. We mustn't take any risks with that. I hope this won't lead to unnecessary delays but it's very important that we always put safety first and we do," Cameron said.

Non-metallic explosives

Al-Qaeda in the Arabian Peninsula long has been fixated on bringing down airplanes with hidden explosives. It was behind failed and thwarted plots involving suicide bombers with explosives designed to be hidden inside underwear and explosives secreted inside printer cartridges shipped on cargo planes.

Over the past year, Americans and others from the West have travelled to Syria to join the fight against the Syrian government. The fear is that fighters with a U.S. or other Western passport, who therefore are subject to less stringent security screening, could carry such a bomb onto an American plane.

People should not overreact to it or over-speculate about what's going on, but there clearly are concerns …. - Homeland Security Secretary Jeh Johnson

The counterterrorism official declined to describe the bomb. But officials in the past have raised concerns about non-metallic explosives being surgically implanted inside a traveller's body, designed to be undetectable in pat-downs or metal detectors.

The call for increased security was not connected to Iraq or the recent violence there, said a second U.S. counterterrorism official who was not authorized to speak publicly by name. Another U.S. official, also speaking on condition of anonymity, said the increased security measures had nothing to do with the upcoming July Fourth holiday or any specific threat.

The extra security is out of an "abundance of caution," the U.S. official said.

"People should not overreact to it or over-speculate about what's going on, but there clearly are concerns centred around aviation security that we need to be vigilant about," Homeland Security Secretary Jeh Johnson said late Wednesday on MSNBC.

Heathrow 3rd busiest

A spokeswoman for Britain's transport ministry declined to give any further details on security measures there, but a witness at London's Heathrow Airport boarding a flight to the United States said shoes, bags and electrical equipment such as laptops were being checked.

Heathrow is the world's third busiest airport and the busiest in Europe, serving 191,200 passengers per day. American Airlines, Delta and United Airlines fly from the airport.

Britain, which lost 52 people when suicide bombers struck the London transport system on July 7, 2005, said its current threat level was 'substantial', a level that means there is a strong possibility of an attack.

That level has been in place since July 2011 when the level was lowered to 'substantial' from 'severe', a level that means an attack is highly likely.

Meanwhile, the State Department has instructed U.S. Embassy employees in Algeria to avoid U.S.-owned-or-operated hotels through July 4 and the Algerian Independence Day on July 5.

"As of June 2014 an unspecified terrorist group may have been considering attacks in Algiers, possibly in the vicinity of a U.S.-branded hotel," according to the message from the U.S. Embassy in Algeria.

250 airports offer nonstops

The U.S. shared "recent and relevant" information with foreign allies, Johnson said in a statement Wednesday. "Aviation security includes a number of measures, both seen and unseen, informed by an evolving environment."

It wasn't clear which airports were affected by the extra security measures, but industry data show that more than 250 foreign airports offer nonstop service to the U.S.

Southwest Airlines, which, along with subsidiary AirTran Airways, flies between the U.S. and Mexico and the Caribbean, doesn't expect the directive to have much impact on its operations, spokesman Chris Mainz said. He said the focus likely would be in other parts of the world, although the airline's security people have been contacted by the Homeland Security Department. Mainz declined to comment on those discussions.

American Airlines spokesman Joshua Freed said the airline has been in contact with Homeland Security about the new requirements but declined to comment further.