CHICAGO- Delta Air Lines is cautious about adding more flights to its schedule due to spikes in coronavirus cases in some parts of the United States, Chief Executive Ed Bastian said in an employee memo on Thursday.

"The continued growth of the virus through the Sun Belt, coupled with quarantine restrictions being implemented in large markets in the northern part of the country, give us renewed caution about further schedule additions at this time," Bastian said.

Delta added about 1,000 flights across its network for July after some modest signs of demand improvement in May and June, but it is still only operating about 30% of what its schedule would normally be.

Shares of Delta, which is due to report second-quarter results on July 14, slipped 3% in early trading.

(Reporting by Tracy Rucinski; Editing by Bernadette Baum) ((tracy.rucinski@thomsonreuters.com;))