WASHINGTON, United States — Domestic flights across the United States (US) have been cleared to resume following an hours-long nationwide ground stop caused by a telecommunications outage, although delays and cancellations are expected to continue.

According to CNN, Transportation Secretary Sean Duffy blamed outdated air traffic control technology and overloaded routers for the disruption, warning that similar incidents could occur unless the system is upgraded.

READ: FAA grounds all flights across US amid communications problem

Duffy urged Congress to provide funding to modernise the country’s air traffic control system, saying the return to normal operations would be slow. However, he said the disruption was not expected to affect Thanksgiving travel in the coming weeks.

Meanwhile, Homeland Security Secretary Markwayne Mullin said the ground stop was not prompted by a threat to the United States and was unrelated to a Microsoft outage earlier this week.

The disruption left an estimated 1.5 million people stranded by flight cancellations, according to data firm Cirium, CNN reported.

Flights already in the air were allowed to land as the ground stop remained in effect for more than four hours, leaving airports across the country facing significant disruption.