Transportation Security Administration chief John Pistole said Thursday there was no timeframe for lifting new security measures at overseas airports triggered by an “ongoing” credible threat targeting airlines.

New procedures, aimed at finding possible cell phone and shoe bombs, went into effect last month following earlier security measures in January.

During a panel at the Aspen Security Conference moderated by Fox News, Pistole also described the latest generation of underwear devices, intercepted in 2012, as using explosives the TSA’s bomb detection dogs were not trained to find. The non-metallic explosive also evaded security screening machines.

That degree of sophistication, Pistole said, showed Al Qaeda in Yemen’s ability to revise its tactics and learn from previous mistakes.

The first underwear bomb, part of a 2009 plot to bring down a flight over Detroit, failed because would-be bomber Abdul Mutullab wore the device for two weeks, causing the explosives to "degrade," Pistole said.

Pistole also said all U.S. airports have extensive perimeter fencing to provide adequate safeguards against manpads – or shoulder fired missiles -- but some “non-compliant” airports overseas remain vulnerable since their perimeters are not large enough or adequately monitored.

Pistole declined to identify those airports, but said the threat from manpads at those sites was one of the aviation vulnerabilities that kept him up at night.

As for the rise of foreign fighters, Pistole confirmed the TSA has the ability to track passengers of interest 72 hours before their travel from airports overseas into the U.S., where they can be identified for a second screening or further investigation.

He said he was less concerned about the nationality and group affiliations of the suspects than he was about their bomb-making capabilities.

The Al Qaeda affiliate in Yemen and its chief bomb maker, Ibrahim al-Asiri, have trained operatives with non-metallic explosives and some of those operatives are believed to be in Syria.

The Aspen Security Forum is a five-day conference that brings together the nation’s most senior present and former government officials from the White House and the Departments of Homeland Security, Defense, State, Treasury and Justice as well as from the U.S. intelligence community and Congress.