Airlines canceled about 600 flights in the United States on Thursday morning after calling off hundreds of flights the previous day, as thunderstorms in Dallas hit operations at one of the busiest airports in the country.

A total of 1,213 flights were canceled across the country on Wednesday, according to flight-tracking website Flightaware.com.

American Airlines Group Inc said "unexpected and heavy" thunderstorms had hit operations at its largest hub at the Dallas Fort Worth International Airport, leading to the cancellation of many flights on Wednesday and Thursday, according to an internal memo seen by Reuters.

The airline canceled 368 flights on Wednesday and had called off nearly 300 more as of Thursday morning, Flightaware.com data showed.

"This was the worst storm we've seen at DFW this summer," American Airlines Chief Operating Officer David Seymour said.

In June, adverse weather conditions and labor shortages had forced several U.S. carriers to cut thousands of flights.

(Reporting by Nathan Gomes in Bengaluru; Editing by Devika Syamnath and Shinjini Ganguli)