Frontier Airlines and Silver Airways are dropping air service to Cuba, saying other airlines are adding too many flights to the island nation and making the routes unprofitable.

U.S. airlines rushed to begin flights to Cuba last year after the Obama administration allowed commercial service for the first time in more than half a century. More Americans are visiting Cuba, but the glut of new flights has exceeded demand, resulting in many empty seats.

Florida-based Silver said Monday that it will end Cuba service on April 22, just six months after it started flying between Fort Lauderdale, Florida, and Cuban provincial capitals like Camaguey and Cienfuegos. Several of its Cuban destinations are also served by major carriers such as American Airlines.

Silver spokeswoman Misty Pinson said the number of seats on planes between the U.S. and Cuba quadrupled because the airlines added so many flights, many of them with big planes. The glut of seats has made Cuban routes unprofitable for all carriers, she said.

Denver-based Frontier Airlines will cancel its daily flight between Miami and Havana on June 4, said spokesman Jim Faulkner, because of heavy competition and higher-than-expected costs of providing service at the Havana airport.

The bigger airlines have not been immune. American reduced daily flights to Cuba from 13 to 10 and switched to smaller planes on some flights. Changes were on flights to smaller Cuban cities; American has not downgraded service to Havana. JetBlue Airways took out about 300 seats a day by using smaller planes.

Commercial air service helped fuel an historic, ongoing boom in U.S. travel to Cuba. About 285,000 tourists visited in 2016, up 76 percent from 2015, and the Cuban government says U.S. visitors increased 125 percent in January.