US air travel reached its highest level since March on Sunday, but still remains about 70 percent below pre-pandemic levels, government statistics showed.

The Transportation Security Administration (TSA) said 831,789 passengers were screened at airport checkpoints, the most since March 17.

A year ago, about 2.647 million passengers were screened on Aug 9.

Airline stocks, including American Airlines, Delta Air Lines, United Airlines and Southwest Airlines rallied on the news, but summer travel remains significantly below pre-pandemic levels.

"As such we remain cautious on the pace of recovery from here as we head into more off-peak leisure travel periods this fall," Andrew Didora, an analyst at Bank of America, said Monday in a research note.

Airlines compete for fewer passengers by cutting ticket prices, further eroding revenue. Lower prices are good news for travelers and bad news for stockholders.

Executives have warned that about 70,000 airline workers could lose their jobs when the first round of federal aid runs out in October. Airlines accepted the aid on the condition that jobs would not be cut.

Airlines and unions are urging Congress to pass an additional aid package totaling $25 billion to carry the industry through March 2021.

Relief for airlines could be included in the next coronavirus aid package, but Congress failed to pass a bill last week. President Donald Trump issued executive orders intended to help workers but didn't include additional aid for airlines.

US Treasury Secretary Steven Mnuchin said the administration is open to resuming negotiations with congressional Democrats and would increase the amount of aid offered as part of the effort to reach a compromise.

"The president is determined to spend what we need to spend," he told CNBC's Squawk on the Street. "We're prepared to put more money on the table."