July 21 (Reuters) - Delta Air Lines ( DAL ) struggled to

restore normal operations on Sunday after last week's crippling

global cyber outage, canceling about 1,000 flights on top of the

3,500 it had already scrapped.

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 ( CRWD )

triggered system problems for Microsoft ( MSFT )

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 ( CRWD ) said on Sunday a significant number of the 8.5

million Microsoft ( MSFT ) devices impacted as a result of a global tech

outage were back online.