Several U.S. airlines on Thursday lowered their financial forecasts, citing weaker bookings amid a rise in Covid-19 cases in recent weeks.

United Airlines said weaker revenue will mean adjusted pre-tax losses in the third and fourth quarters of this year. The Chicago-based carrier in July said it expected to post pre-tax profits for that period. It plans to further trim capacity this year because of weaker demand.

American Airlines said its third-quarter sales will likely be down by as much as 28% compared with the same quarter of 2019. The company had previously forecast a 20% drop in revenue. American on Thursday also said it expects deeper negative profit margins. Southwest Airlines said it is also logging weaker bookings and higher cancellation rates.

Air travel generally falls in late summer as schools reopen, but airline executives in recent weeks have warned that the fast-spreading delta variant has exacerbated the drop in demand.

Covid cases and further delays for companies’ plans to return to offices has cast doubts on business travel demand, a lucrative segment of air travel that usually picks up in the fall.