Americans should be alert to the possible travel risks, especially during the holidays, following increased terrorist threats around the world, the State Department warned on Monday.

A travel alert, which is to be in effect until Feb. 24, said current information suggests that militants with the Islamic State, al-Qaida, Boko Haram and other terrorist groups continue to plan attacks in multiple regions. U.S. authorities said the likelihood of terror attacks will continue as members of IS return from Syria and Iraq, and other individuals not affiliated with terror groups engage in violence on their own.

Extremists have targeted sporting events, theaters, open markets and aviation targets. In the past year, there have been multiple attacks in France, Nigeria, Denmark, Lebanon, Turkey and Mali. IS has claimed responsibility for the Oct. 31 bombing of a Russian airliner in Egypt, killing 224 people.

"U.S. citizens should exercise vigilance when in public places or using transportation," the alert said. "Be aware of immediate surroundings and avoid large crowds or crowded places. Exercise particular caution during the holiday season and at holiday festivals or events."

The State Department said the U.S. is exchanging information with allies about threats of international terrorism.

The travel alert was issued the same day that Belgium's prime minister announced that Brussels would remain at the highest alert level for at least another week. The increased security measures following the Nov. 13 attacks in Paris that killed 130 people have virtually shut down the Belgian capital.

At Orlando International Airport on Tuesday morning, travelers said the alert has them on edge, but they said it won't stop them from getting from Point A to Point B this holiday week.

"It's scary knowing some places you can't be safe in," said Allyson Futch, a college student who lives in the Orlando area.

Melody Ramirez, who was traveling to Los Angeles, added: "It's something to keep in mind, but I don't want to make that relevant to my travels."

Orlando International Airport is expected to be the seventh-busiest airport in the country this week, with about 1.5 million people passing through the terminals.

Airport officials have stepped up security measures, but on Monday, 20 passengers on Norwegian Flight 7055 bypassed customs after the main terminal gate was left open.

U.S. Customs and Border Protection released the following statement Tuesday:

U.S. Customs and Border Protection identified an infraction involving the processing of a Norwegian Air flight from Copenhagen, Denmark, that arrived at Orlando International Airport. Approximately 20 passengers inadvertently exited through a door to the domestic gate. CBP officers immediately responded to the incident and ensured all passengers and their luggage were processed through the federal inspection area before leaving the airport. CBP is investigating the incident with the airport authority and the airline and will implement measures to prevent similar occurrences in the future.

Steven Jones, who was visiting Central Florida from Detroit, said the country has to trust the government's security measures.

"You just have to go about your daily business and not let them win," he said. "Just put it behind you and go about your daily business."

Airport security officials said no threats have been made toward Orlando International Airport.

— News 13 Reporter Bailey Myers contributed to this report from Orlando International Airport.