British airports have joined the US in stepping up security following fears that terrorists from Syria and Yemen are developing bombs that could be smuggled on to aircraft.

The Department of Transport (DfT) said it has taken the decision to step up "some of our aviation security measures" after reciving warning from US security chiefs.

US Homeland Security Secretary Jeh Johnson said there will be "enhanced security measures" at selected airports with direct flights into the United States, but did not state which ones.

The warning comes amid growing unrest in Iraq and Syria and ahead of the US's July 4th Independence Day celebrations.

A UK DfT spokesperson said the increased security is not expected to cause "significant disruption" to passengers and the threat level remains unchanged at "substantial" -- meaning there is a strong possibility of attack. The threat level has been at substantial -- in the middle of the five threat levels -- since July 2011.

A spokesperson added: "The UK has some of the most robust aviation security measures and we will continue to take all the steps necessary to ensure that public safety is maintained."

The DfT did not comment on what extra precautions will be implemented.

US officials said the alert was triggered following intelligence reports that al-Qaeda terrorists in Syria and Yemen had joined forces to develop undetectable explosive devices designed to avoid airport screening.

It is believed one of the suggested bomb-makers are the Yemen-based al-Qaeda in the Arabian peninsula (AQAP), who were behind the so-called failed "underwear bomb" which breached airport security but failed to bring down a Delta Air Lines jet over Detroit in 2009.

It is reported the terrorist groups are attempting to build non-metallic bombs which could be surgically implanted inside a body, designed to be undetectable in pat-downs or metal detectors.

Johnson added: "We are sharing recent and relevant information with our foreign allies and consulting the aviation industry. These communications are an important part of our commitment to providing our security partners with situational awareness about the current environment and protecting the travelling public."

Deputy Prime Minister Nick Clegg said the new security measures will not just be a "blip for a week".

He told LBC: "This is part of an evolving and constant review about whether the checks we have in airports keeps up with what we know from intelligence about the nature of the threats we face.

"People should not be unduly panicked or concerned but this is the world we now live in. This won't be the last time that further adjustments are made because we are having to constantly evolve our own defences in view of the evolving way people want to attack us."