June 21 (Reuters) - Delta Air Lines Inc (DAL.N) aims to hire more than 1,000 pilots by next summer, according to a company memo reviewed by Reuters on Monday. Several other U.S. carriers have also said they plan to hire more pilots and staff. Delta expects U.S. leisure travel volume this month to return to pre-pandemic levels and is seeing more business travelers return to the skies, Chief of Operations John Laughter wrote to operations employees. On Sunday, the U.S. screened 2.1 million air travelers -- the highest number since March 2020 when COVID-19 slashed demand but still down 23% from pre-pandemic levels. Airlines are preparing for more travelers. After heavy losses in 2020, Delta has said it expects to generate a pre-tax profit in the second half of 2021, with a re-opening of corporate America by Labor Day in early September. "The fact that we expect to record a profit in June - just 15 months after the sharpest decline in aviation history - is remarkable," Laughter said in his employee note. He sounded a note of caution on the timing for building back its international network, however, but cited "welcome openings in markets like Spain, France, Italy and Greece." The Atlanta-based carrier anticipates travel restrictions easing across the Atlantic in the second half of 2021, Laughter added. American Airlines (AAL.O) said Monday it intends "to resume pilot hiring this fall with approximately 300 new pilots joining us by the end of the year and double that number in 2022." Southwest Airlines (LUV.N) still has about 500 pilots participating in its voluntary leave program but the airline said Monday it "anticipates hiring first officers" later this year "to support the airline’s 2022 operations and scheduled aircraft deliveries." Chicago-based United Airlines (UAL.O) aims to start adding some 300 new pilot hires in the coming weeks, but hiring beyond that would depend "to some degree on the speed at which we recover from the pandemic," a spokesman said. United more broadly aims to hire some 10,000 pilots by 2030, the spokesman, Charles Hobart, added.