Airlines scramble to add Caribbean flights after airspace restrictions strand tens of thousands of travelers

Airlines scrambled to add dozens of extra flights for tens of thousands of stranded travelers throughout the Eastern Caribbean on Sunday after the Federal Aviation Administration's flight restrictions in the area though some customers found no seats available for days.

The FAA had closed airspace in the region to U.S. commercial flights after the United States' strikes in Venezuela. The restrictions expired overnight, allowing flights to resume.

The flight curbs forced airlines to cancel hundreds of flights in the region on Saturday, stranding customers at the end of the busy holiday period.

FlightAware showed about 20 cancellations in San Juan on Sunday, compared with 400 a day earlier. However disruptions for some travelers could last for days because seats were scarce and previously scheduled flights were packed for the end of the New Year holiday weekend and school vacations.

Airlines received complaints from some users on X that they couldn't find available seats back to the U.S. until the end of the week. Some said they didn't have places to stay or couldn't afford additional nights at Caribbean hotels.

American Airlines told CNBC it added 17 extra flights between San Juan, Puerto Rico, as well as Aruba, the U.S. and British Virgin Islands, Antigua, Barbados, Dominica and Barbados, and its hubs in Miami and Charlotte, North Carolina. Southwest Airlines added six extra round-trips between San Juan on Sunday and another eight on Monday as well as two additional flights to Aruba on Sunday.

"We are looking for opportunities to add more capacity to both places in the coming days," the airline said in a statement.

But as of Sunday morning, even new flights were listed as unavailable on airline websites, as seats sold out quickly in the demand surge.

United Airlines and Delta Air Lines plan to add additional flights on Sunday. Carriers were evaluating using larger planes, like those normally used for Europe or Asia, to accommodate the surge in demand. 

American said it would use two Boeing 777-300s, the largest aircraft in its fleet and that sits 304 passengers, for two San Juan-Miami roundtrips on Monday.

Airlines generally add flights and send bigger planes ahead of evacuations for hurricanes but had little time to prepare after the United States' attack in Venezuela.

The airlines waived change fees and fare differences for customers affected by the closures if they rebooked flights later in the month. More than a dozen airports in the region were included in the waivers.

Major U.S. airlines have not served Venezuela directly for years. American Airlines was the last major carrier to halt flights to the country in 2019 amid unrest.

The temporary restrictions were set to expire at midnight ET, U.S. Transportation Secretary Sean Duffy said in a post on X.

The United States' attack on Venezuela resulted in the capture of Venezuelan President Nicolas Maduro and his wife. Maduro was indicted on narco-terrorism conspiracy and other charges in the Southern District of New York. His indictment was unsealed and posted online by U.S. Attorney General Pam Bondi on Saturday.

The flight restrictions after the U.S. attacks underscored how quickly military action can disrupt civilian air travel, forcing airlines to suspend operations well beyond the immediate conflict zone.

Airlines have been forced to take longer routes to avoid airspace if not periodically halt flights altogether due to conflicts in the Middle East as well as after Russia's invasion of Ukraine in 2022.

-- CNBC's Victor Loh contributed to this article.