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

Sunday to restore normal operations after last week's crippling

global cyber outage, canceling 1,250 flights beyond the 3,500 it

had already scrapped.

The issue has stranded thousands of Delta travelers across

the United States, with some having to rent cars to drive

hundreds of miles while others could have to wait days for new

flights or cancel trips altogether.

The Atlanta-based airline, which canceled a third of its

schedule and delayed another 1,700 flights or 44%, according to

FlightAware, is battling operational issues after the outage hit

its crew tracking system.

Delta has offered no timetable for resumption of normal

operations and has already canceled another 305 flights for

Monday, the flight tracking website said. Its total of canceled

flights since Friday now stands at more than 5,000.

A software update by global cybersecurity firm CrowdStrike ( CRWD )

triggered system problems for Microsoft ( MSFT )

customers, including many airlines, on Friday.

Although other U.S. airlines have largely recovered, Delta

has struggled to return to normal. United Airlines

canceled 9% - or 266 - of Sunday's flights, the second most

among carriers.

Delta CEO Ed Bastian said the issue affected 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.

In a separate note, he told employees that Delta would

continue to "tactically adjust" schedules to ensure safety.

Transportation Secretary Pete Buttigieg spoke with Bastian,

according to an official, reminding him of the carrier's

responsibilities to customers and the department's enforcement

role.

"I will ensure that our department supports Delta passengers

by enforcing all applicable passenger protections," Buttigieg

said in a statement to Reuters, adding that the department had

received hundreds of complaints.

"No one should be stranded at an airport overnight or stuck

on hold for hours."

CrowdStrike ( CRWD ) said a significant number of the 8.5 million

affected Microsoft ( MSFT ) devices were back online.