All flights from several major US airlines – including Delta, United and American Airlines – were grounded Friday morning due to a communication issue, according to the Federal Aviation Administration.

The ground stop impacts all flights from the airlines, regardless of their destination, said the FAA.

It’s unclear how long the ground stop will last, though the FAA suggested an update would be available by 5 a.m. ET.

Their grounding comes after a significant Microsoft outage brought Frontier Airlines to a standstill for hours on Thursday – though the ground stop for those airlines has since been lifted.

Earlier Thursday, Frontier said its systems had been impacted by the outage, and offered refunds to inconvenienced passengers.

Other competitors including Allegiant and SunCountry also said they were having difficulties, including with their booking, check-in and trip-managing functions online.

The FAA also announced Friday morning all Allegiant flights would be grounded.

On the Azure cloud software status report site, Microsoft said that service went down for some customers in the central United States around 6 p.m. ET, “including failures with service management operations and connectivity or availability of services.”

The company said it determined the cause and is working to fix it. A company spokesman did not immediately respond to a request for comment.