US orders tighter airport security amid evolving threats

US-bound travellers from Europe and the Middle East have faced tighter airport security, due to fears that Muslim extremists are developing new explosives that could be slipped onto planes undetected.

The stepped-up security checks were ordered as the US embassy in Uganda warned of a "specific threat" to attack Kampala's Entebbe international airport today between 1800 and 2000 GMT (6am and 8am NZ time).

Although the embassy did not name any group, Al-Qaeda linked Shebab insurgents have claimed recent attacks in neighbouring Kenya and Djibouti, and at home in Somalia.

US Homeland Security Secretary Jeh Johnson announced the extra security on direct flights to the United States from some overseas airports yesterday, without citing evidence of any specific plot.

The move comes amid broader Western intelligence concerns that hundreds of Islamist radicals travelling from Europe to fight in the Middle East could pose a security risk on their return.

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 without a visa.

The airports concerned 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.

Analysts said the move was likely linked to concerns Al-Qaeda in the Arabian Peninsula (AQAP) was passing on bomb making expertise to militants fighting in Syria.

The fear is that militants with European passports could then bring such skills back home with them and launch an attack, experts add.

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.

The extra security could create more travel delays but applies mostly to airports in Europe, Africa and the Middle East and the US directive does not include New Zealand.

An Auckland Airport spokesman said authorities were aware of the notice but had not been asked to step up security.

Air New Zealand said it was "ready to address any new requirements as necessary to remain compliant".

- Additional reporting AP