Heathrow Airport
US officials have not revealed which airports will be subject to the enhanced security measures.Wiki Commons

The United States is stepping up security at overseas airports which have direct flights to the US.

The latest security measures come amidst heightened concerns that terrorists are developing sophisticated, "undetectable" explosives, designed to avoid airport screening.

US Homeland Security Secretary Jeh Johnson said in a statement that he has directed the Transportation Security Administration to "implement enhanced security measures in the coming days" at selected overseas airports.

"We will work to ensure these necessary steps pose as few disruptions to travellers as possible," he said.

"We are sharing recent and relevant information with our foreign allies and are consulting the aviation industry."

The department did not specify which airports or what countries would be affected, nor did it say what has prompted the enhanced measures.

Reuters reports that, following the unsuccessful attempt by the so-called "underwear bomber" to bring down a Delta Air Lines jet over Detroit in 2009, US counter-terrorism officials had expressed concerns that al Qaeda operatives in Syria and Yemen had teamed up to develop undetectable bombs that could be smuggled onto planes.

ABC News has reported that the explosives these operatives are attempting to build include non-metallic bombs.

After the 9/11 al Qaeda attacks, immediate measures were taken to harden cockpit doors. The U.S. aviation security has since shifted its focus from hijacking, to plastic explosives that can be carried aboard a plane or hidden in baggage.

Officials also said that the enhancements have not been prompted by any specific plot, and the matter was not related to violence, developments in Iraq and Syria or the July 4 holiday weekend.

Security officials were not considering any new restrictions on what passengers can bring aboard flights.