Delta Air Lines expects to add about 1,000 flights in August but not many more for the remainder of the year, Chief Executive Ed Bastian said in an internal memo on Thursday that mentioned a resurgence of COVID-19 in parts of the country.

Airlines including Delta have added back some summer capacity after modest improvements in domestic travel demand in May and June, but analysts say a spike in cases in some states, which have prompted three Northeast states to order quarantines for some travelers and Disneyland to delay its reopening, could dampen the recovery.

Bastian said Delta continues to see slow but steady increase in passenger volumes two weeks into the summer travel season, but warned that the airline expects overall demand this summer to be only 25% of last summer's revenue.

"While it's encouraging to see flights returning (...) we likely remain at least two years away from a return to normal," Bastian said.

To prevent the spread of the novel coronavirus on airplanes, US airlines have recently cracked down on passengers who do not follow new requirements to wear face coverings by threatening to remove their future flight privileges.

Bastian said the airline has already banned some passengers from future travel on Delta for refusing to wear masks on board.