The administration appears to be significantly worried about another attack from al-Qaeda that would target passenger planes flying to the US from overseas.

BBC:

France has become the latest country to increase airport security in line with a request from Washington to enhance screening for flights to the US.

On Wednesday the US warned of a "credible" terrorism risk to international flights, without revealing the specifics of the threat.

The UK and Germany are among those who have complied with the request.

On Thursday security was increased in Uganda's capital Kampala following a "specific threat" to its airport.

France's civil aviation authority, DGAC, said the enhanced screening "will be carried out in a way to limit as much as possible inconvenience to passengers".

It warned however that "delays are possible".

A DGAC spokesman said "we cannot divulge the added measures" being taken.

Dozens of direct flights from France to the US are believed to be affected. Officials are recommending passengers arrive at airports early to allow time for screening.

On Thursday the UK said that it too would increase checks because of an "evolving threat" but said people should travel as usual.

'Credible threat'

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

What has US authorities so worried is the proliferation of western jihadists going to Syria and that al-Qaeda's bomb making expertise has migrated there as well:

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.

The terrorists may target several planes at once, hoping to suceed with blowing up at least one of them before we can figure countermeasures to the new bomb.

I'd think twice about taking a European vacation this year.