Several U.S. airline executives this week said they were prepared to push back if regulators seek to make Covid-19 testing a requirement for domestic flights, though they also said they have seen no indication that is coming.

A U.S. Centers for Disease Control and Prevention requirement went into effect this week requiring all inbound passengers to test negative for Covid-19 prior to boarding. In an earnings call this week, American Airlines chairman and CEO Doug Parker said the requirement has caused a hit on international demand, particularly for short-haul travel, such as to Mexico and the Caribbean. In general, however, he expressed support for the new measures.

“We support international testing, because that’s about getting more people to be comfortable flying across borders,” Parker said. “We are hopeful that doing so allows [President Joe Biden’s] administration to get more comfortable with airlines and be open to allowing more people to travel to the United States at some point.”

American Airlines chief customer officer Alison Taylor also told BTN that the week largely went smoothly in terms of the new requirements, with only a small number of travelers showing up for flights without having met them.

This week, Reuters cited a CDC official as saying the Biden administration is “actively looking” at expanding that requirement to include testing before domestic flights. Both Parker and Southwest Airlines chairman and CEO Gary Kelly said in their respective earnings calls that they have not heard directly heard from regulators floating that possibility. Were it to come, however, they said it would be difficult to implement.

“I would just make the argument: Why pick on air travel?” Kelly said. “If you want to test people, test them, but test them before they go to the grocery store. Test them before they go to a restaurant. Test them before they go to a sporting event.”

JetBlue president and COO Joanna Geraghty had a similar take in the carrier’s earnings call, saying it was “far too cumbersome” to put the burden on air travel when travelers could also tap alternatives including trains and buses. In addition, adding a testing requirement for domestic flights would add more stress to already overburdened labs across the country trying to process tests in a timely fashion.

“Frankly, we’re concerned that it would actually reduce the ability of some people who legitimately need to get tested for health reasons to get tested,” Geraghty said.

Parker said American is prepared to comply with whatever regulations come along, but he said domestic testing would be “difficult” and unnecessary given studies that have indicated Covid-19 transmission on aircraft is limited.

“We’ll obviously work with the administration and what they think makes sense and do our best to make sure the world doing everything we can to make sure that people are safe, and also that we get through this pandemic as quickly as possible, but we’ll also let them know what kind of impact that would have on travel,” Parker said. “I didn’t say that we weren’t supportive of doing something more, but if we do, we certainly would want to make sure it was something that wouldn’t restrict demand.”