"We ask that passengers follow the instructions of international security and flight crews, " the TSA said.

CNN: Security ramped up at airports in U.S., UK, Amsterdam

The rule prohibited passengers on international U.S.-bound flights from leaving their seats during the last hour of a flight, the sources said.

CNN: Canada limits carry-on baggage on flights into U.S.

However, if no air marshals are aboard (currently they are assigned to most international flights but only a small percentage of domestic flights), flight attendants and passengers are the first line of defense.

FORBES: A How-to, When-to Guide For Airplane Vigilantes

The Transportation Security Administration invoked a "one hour rule" after the thwarted attack, which prohibited passengers on international U.S.-bound flights from leaving their seats during the last hour of a flight, sources said.

CNN: Air travel security under review, officials say