US flights were slowly resuming departures and a ground stop was lifted after the Federal Aviation Administration (FAA) scrambled to fix a system outage overnight that had forced a halt to all US departing flights but the reason for the suspension remained unknown.

Iran PressAmerica:  Thousands of flights were canceled or delayed after a government system used to give pilots safety and other information broke down overnight.

More than 7,300 flights were delayed and 1,100 canceled according to the FlightAware website in the first national grounding of flights in about two decades, industry officials said.

The total was still rising and officials said the outage could cause delays through at least Thursday, if not longer, according to several airlines.

The cause of the problem with a pilot-alerting system was unclear, but US officials said they had so far found no evidence of a cyberattack. US Transportation Secretary Pete Buttigieg told CNN an issue with overnight "irregularities" with safety messages sent to pilots prompted the outage.