(CNN) -- Homeland security officials could as soon as Wednesday announce new aviation security measures for flights headed to the United States, reflecting heightened concern that terrorists are developing more sophisticated explosives designed to avoid airport screening, U.S. officials told CNN.
Officials said there is no specific plot and the matter is not related to violence and other developments in Iraq and Syria or the July 4 holiday weekend.
Potential new measures would involve increased screening for U.S.-bound flights that take off from Europe and the Middle East, according to one official, while another said the measures would be related to airports worldwide that don't meet American security standards.
New restrictions on what passengers can bring aboard flights are not being contemplated.
Since hardening cockpit doors and taking other measures following the 9/11 al Qaeda attacks, the U.S. aviation security focus has shifted primarily from hijackings to plastic and other explosives that can be carried aboard a plane or hidden in baggage.
The United States has particularly been focused on efforts by al Qaeda in the Arabian Peninsula to develop undetectable explosives since the unsuccessful attempt by the so-called "underwear bomber" to bring down a Delta Air Lines jet over Detroit in 2009.
ABC News first reported Wednesday the Obama administration was planning to move forward with efforts to step up security at overseas airports, and was "likely to issue a series of directives" in coming weeks to airport authorities and airlines to expand passenger screening if they want to serve U.S. cities.