Some airlines are already suspending flights and are not offering refunds to passengers for flights to destinations outside of the United Kingdom.

The US Department of Transportation (DOT) says that American Airlines, Delta Air Lines, American Airlines Alaska, Southwest Airlines, US Airways and United Continental Holdings are among those affected. 

The US Department also says that JetBlue, United Airlines, and American Airlines are suspending flights in some regions, including New York City, Washington DC, and Los Angeles.

American Airlines also announced that it is suspending all flights between Los Angeles and San Francisco on Tuesday.

JetBlue says that it will resume flights between New York and Boston on Wednesday. 

American Airlines, which operates over 1,000 flights a day, has not released any travel advisories or travel restrictions for the US. 

“The Department of Homeland Security continues to closely monitor the situation, and will continue to provide updates as warranted,” a statement on American Airlines website reads.

“The Department is working closely with our airlines, partners and customers to provide our customers with the necessary information and assistance.”