LONDON : Travellers flying to the United States from Europe and the Middle East face tighter airport security checks from Thursday due to fears extremist Muslim groups are making new explosives able to slip by standard checks.

The measures threaten fresh disruptions for passengers at the start of the summer holiday season and will affect some direct flights to the US.

Officials declined to say if they had uncovered a specific plot.

But the move comes amid increased concern among Western intelligence services that Islamist fighters who go to Syria with European passports could pose a security risk on their return.

The announcement of the extra checks on direct flights from some overseas airports to the US was made by US Homeland Security Secretary Jeh Johnson Wednesday.

“We are sharing recent and relevant information with our foreign allies and are consulting the aviation industry,” Johnson said in a statement.

The airports are located in the Middle East and Europe and were targeted “based on real-time intelligence”, according to an official at the Department of Homeland Security who spoke on condition of anonymity. Britain confirmed it is bolstering security at its airports in response.

The move came before the US Independence Day celebrations on Friday. On Sunday, US President Barack Obama warned that “battle-hardened” Europeans who embrace jihad in Syria and Iraq threaten the United States because their passports mean they can enter his country without a visa.

Despite the increased checks, Britain said the international terror threat level issued by security service MI5 remained unchanged at substantial, the third highest grade out of five, where it has been since July 2011.

Deputy Prime Minister Nick Clegg said that extra security measures were the right response to “credible new threats”.

“It’s very important that we work - as we do - with our American partners and indeed with other countries around the world so that where credible new threats are identified, a response is then implemented in airports around the world,” Clegg told ITV television.

Transport Secretary Patrick McLoughlin insisted to Sky News that passengers should not face “significant delays”.

The Department for Transport did not comment in detail on the nature of the security searches.

London’s Heathrow airport - the world’s busiest international air hub - said that to reveal the details “would be very helpful for the people who want to pass through the system”.

Aviation security expert Philip Baum told Sky the added checks would likely mean an increase in random searches and the number of passengers being asked to remove their shoes at security.

Passengers in Britain have long faced tight security measures at airports following high-profile threats including a failed attempt by British “shoe bomber” Richard Reid to blow up a US-bound flight in 2001.

Security was further tightened after a plot to blow up “liquid bombs” on transatlantic flights was uncovered in 2006.

Ben Friedman, a defence and homeland security expert at the Cato Institute think-tank in the US, told the BBC that fears had been raised over people with European passports who were fighting in Syria.

But he noted that previous attempts by Yemeni bomb makers to blow up planes had been unsuccessful.

“While we want to be prudent and cautious... these guys are serial failures,” he added.

A Nigerian, Umar Farouk Abdulmutallab, is serving a life sentence for trying to blow up a US-bound airliner on December 25, 2009 with explosives hidden in his underwear.

US officials said the botched plot was the work of Al-Qaeda in the Arabian Peninsula, based in Yemen.