Two major U.S. airlines are offering refunds to passengers worried about the Zika virus outbreak in many tropical countries.

United Airlines says customers booked to fly to areas affected by the virus can reschedule or get refunds. American Airlines says it will give refunds to pregnant women who were planning to travel to parts of Central America.

The U.S. Centers for Disease Control and Prevention have warned pregnant women to take precautions against mosquito bites when traveling to areas in Latin America and the Caribbean where there have been Zika outbreaks. The CDC says the mosquito-borne illness could be linked to a birth defect of the brain.

The United Airlines offer began Tuesday and includes any country covered by a CDC travel notice, an airline spokesman said. American Airlines began refunds Monday for pregnant passengers holding tickets to El Salvador, Honduras, Panama or Guatemala, according to a spokesman.

A spokesman for Delta Air Lines said the carrier was monitoring the situation but not yet offering waivers. JetBlue Airways and Spirit Airlines were not immediately able to say whether they were offering refunds. Southwest Airlines said it was sticking to its normal policy, which lets customers who cancel ahead of time reuse the value of their tickets. All of those airlines fly to at least some affected locations.

On Tuesday, the CDC expanded its travel alert for pregnant women to add the U.S. Virgin Islands and the Dominican Republic to the list of areas with Zika outbreaks. The CDC has already recommended that pregnant women consider postponing trips to 22 other destinations.

— In Central and South America: Bolivia, Brazil, Colombia, Ecuador, El Salvador, French Guiana, Guatemala, Guyana, Honduras, Mexico, Panama, Paraguay, Suriname and Venezuela.

— In the Caribbean: Barbados, Guadeloupe, Haiti, Martinique, St. Martin and Puerto Rico.

— And Cape Verde, off the coast of western Africa, and Samoa in the South Pacific.