US officials halted flight departures nationwide on Thursday over safety concerns caused by a telecommunications outage, leaving an estimated 1.5 million travellers stranded by cancellations.
Homeland Security Secretary Markwayne Mullin said there was no known timetable for resolving the problem and warned that the process could be lengthy.
An airline source told CNN that a problem with network routers had cascaded across the country. Another source said major airlines could shut down for the evening.
Flights already in the air continued to land, while departures remained grounded.
Mullin said the disruption did not stem from a threat to the United States and was not linked to a recent Microsoft outage. Law enforcement sources told CNN that preliminary reports indicated it was not a cyberattack.
President Donald Trump has been briefed on the situation.
