Delta Air Lines, one of US's largest, faces widespread flight cancelations after IT outage

The Atlanta-based airline, which canceled just over a quarter of its schedule Sunday and delayed another 1,700 flights or 46%, is facing ongoing operational problems caused by the outage's impact on its crew tracking system.

The issue has stranded thousands of Delta travelers across the country, with some resorting to car rentals to drive hundreds of miles and others facing days for new flights or canceling trips altogether.

Delta has offered no timetable for when normal operations would resume and has already canceled another 137 flights for Monday, according to FlightAware.

A software update by global cybersecurity firm CrowdStrike triggered system problems for Microsoft customers including many airlines on Friday.

While other U.S. airlines have largely recovered, Delta has struggled to return to normal. United Airlines canceled 9% -- or 262 -- of its flights on Sunday, second most among carriers.

Delta CEO Ed Bastian said the issue impacted its Microsoft Windows systems, snarling a critical application.

"In particular one of our crew tracking-related tools was affected and unable to effectively process the unprecedented number of changes triggered by the system shutdown," Bastian told customers in an email.

He told employees in a separate note on Sunday that Delta "will continue to tactically adjust our schedule as needed to ensure the safety of our operation."

Transportation Secretary Pete Buttigieg spoke with Bastian on Sunday, according to an official, reminding Bastian of the carrier's responsibilities to customers and the department's enforcement role.

CrowdStrike said on Sunday a significant number of the 8.5 million Microsoft devices impacted as a result of a global tech outage were back online.