ATLANTA - The federal government is preparing a travel advisory instructing Americans to avoid nonessential travel to Mexico, the acting head of the Centers for Disease Control and Prevention said today.

Dr. Richard Besser made the disclosure during a news conference in Atlanta, saying the advisory was being released "out of an abundance of caution."

Besser also reported 40 confirmed cases of swine flu in the United States, including 20 in New York City. He said people can help keep the disease from spreading by taking everyday precautions such as frequent handwashing, covering up coughs and sneezes, and staying away from work or school if they're not feeling well.

Before the CDC changed its advice to travelers, U.S. airlines were reporting that some passengers have already changed or canceled their plans to fly to Mexico.

Spokespeople for US Airways, American Airlines and Delta Air Lines said today passengers have requested travel changes, but none of the carriers would say how many. The three airlines said their operations are proceeding as normal and they have not canceled any flights to Mexico as a result of the scare.