Media playback is unsupported on your device Media caption Frank Gardner reports on increased airport security following a "credible terror threat"

Security is being tightened at airports with direct flights into the US - including some in the UK - in response to US warnings of a "credible threat".

UK Transport Secretary Patrick McLoughlin said the measures were being taken to keep the public safe.

While he would not specify what steps would be involved, he ruled out "significant disruption" to passengers.

It comes amid US media reports that al-Qaeda affiliates in Syria and Yemen are developing bombs to smuggle on planes.

A US Department of Homeland Security (DHS) official said the changes were a response to a "real time" and "credible" threat, but he could not comment on specific intelligence matters.

'Remain vigilant'

DHS Secretary Jeh Johnson said in a statement: "We are sharing recent and relevant information with our foreign allies and are consulting the aviation industry."

The changes are expected in the coming days.

Former US secretary of state Hillary Clinton said she took the threat seriously, saying: "We have to remain vigilant."

Media playback is unsupported on your device Media caption Hillary Clinton: "We have to remain vigilant"

Mr McLoughlin told the BBC that "very stringent" measures were already in place, but that the UK had to take action when given information and advice to do so.

It is thought the measures could include more thorough screening of passengers, checks of shoes and electronic devices.

At Manchester Airport, it is believed extra swab machines were sent to departure gates on transatlantic flights to allow staff to swab hand luggage immediately before boarding, as well as at security.

The prime minister's spokesman said people should continue to fly, but advised they allow "appropriate time" to go through the tighter security.

What changes will we see?

by Richard Westcott, BBC transport correspondent

Image copyright Getty Images Image caption The unspecified measures will be put into place in some airports with direct US flights

The UK government isn't giving any details about what these security changes actually are, but they haven't ruled out longer queues at security.

The statement from the Department for Transport merely says that the majority of passengers should not experience significant disruption.

What we do know is that all of the old rules remain in place. So you will still have to put liquids into separate, see-through bags, take your laptop out of your bag, take off your belt and maybe your shoes before going through security.

These changes are being brought in for US-bound flights but the reality is they'll affect all passengers going through security.

Guards won't just single out people going to the States, it would take too long.

There were longer queues at Manchester Airport this morning because of the changes. But I am told that's because it's new, and things should get back to normal once everyone gets used to the new regime.

The actual terrorism threat level remains the same, at substantial. That's the middle of the five threat levels and means that an attack is a strong possibility.

'Business as usual'

Media playback is unsupported on your device Media caption Transport Secretary Patrick McLoughlin says the UK has "acted on advice and information received"

Some 80 commercial airline passenger flights are departing the UK bound for US destinations on Thursday, according to the National Airport Traffic Services (Nats), which only covers 15 main UK airports and does not take into account cargo, business or private flights.

Heathrow, Stansted, Gatwick and Bristol airports have said they are operating as business as usual.

Al-Nusra Front, al-Qaeda's affiliate in Syria, and Yemen-based al-Qaeda in the Arabian Peninsula (AQAP) are believed to be working together to try to develop explosives that could avoid detection by current airport scanners.

The Arab Spring upheavals of 2011 have left much territory in Yemen ungoverned, giving the AQAP the opportunity to move in and carry out attacks on government positions there.

Yemen also has become one of a handful of countries where the US acknowledges using drones which have killed large numbers of jihadist plotters.

What is the security risk?

by Frank Gardner, BBC security correspondent

Image copyright PA Image caption Longer queues are a possibility

This new increased threat warning has been triggered by a fear amongst western intelligence agencies that some of al-Qaeda's sophisticated bomb-making expertise has proliferated out of Yemen to Syria.

For the last five years jihadists in Yemen have been working on so-called "artfully-concealed devices" - hard-to-detect explosives that contain no metal and emit only a faint vapour.

Three times now they have been able to smuggle these onto international flights. Only one exploded, killing the man carrying it but nobody else, after the plane landed.

There is equipment in place to detect such devices at most major UK airports but it is not used on every passenger.

What is alarming the US Department for Homeland Security is the possibility that jihadists with European passports are now in Syria, learning how to construct such devices before returning home.

'Serial failures'

Ben Friedman, an expert in defence and homeland security at the Cato Institute in the US, told the BBC it was important to remember that attempts by the Yemeni bomb-makers in the past to blow up planes had failed.

He referred to the Christmas Day underwear bomber, who failed to take down a US-bound flight in 2009, and a failed attempt in which a bomb was hidden in a printer cartridge in a cargo plane in 2010, which was also bound for the US.

"These guys are serial failures," he said.

Meanwhile, a picture of home-made bombs was posted on Twitter on Wednesday by a person claiming to be Nasser Muthana, from Cardiff, who appeared in an Isis propaganda video released last week.

He tweeted next to the image: "So the UK is afraid I come back with the skills I've gained."

Image copyright PA Image caption Authorities say they will work to limit disruption for passengers

Body scanner

Security experts told the BBC the best defence against the type of bombs al-Qaeda is thought to be developing was a combination of two things - both a body scanner and an ion body scanner.

The former reveals concealed devices - even those hidden inside a bomber's body - and the latter detects the tiniest particles of explosive residue.

Both devices are used at most major UK airports, but are used on only a minority of passengers, alongside more common measures including X-ray scans, pat-downs and swabs.

Airports across the world ramped up security measures in the aftermath of the 9/11 terror attacks in the US. Cockpit security was also enhanced.

They introduced security checks on footwear after a shoe bomb nearly brought down a plane a few months later.

Restrictions on liquids in hand luggage were introduced in 2006 after a British plot to blow up seven US and Canada-bound flights with liquid bombs was foiled by the security services. Three men behind the terror plot were jailed for life.

Thwarted AQAP plots