WASHINGTON (WABC) -- The Obama administration stepped up security at major transit hubs across the country after Tuesday's airport and subway bombings in Brussels, as top U.S. intelligence officials warned of the risk for copycat attacks at home.
The State Department issued a travel alert for Americans traveling to or through Europe.
"Terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants and transportation," it said in a statement.
Officials advised U.S. citizens to "exercise vigilance" in public places or on mass transportation and to take "particular caution" during religious events or large events.
Passengers say it's unavoidable, the fear that the next wave of attacks may be imminent, whether in Europe or here in the United States.
But New York Senator Charles Schumer says Americans appear to be more vulnerable overseas.
"There is no credible threat aimed at New York City or at the United States. The focus of ISIS up to now has been in Europe," said Schumer.
For that reason, the State Department issued its travel alert for U.S. citizens traveling in Europe.
Many experts now believe the Belgium bombings are only the latest in a series of attacks plotted by members of the Islamic State, and Western intelligence is determined to detect and disrupt whatever comes next.
Security was tightened further Wednesday at major U.S. airports, including JFK, where anxious passengers were grateful for the increased vigilance and the travel alert.
"I think it's correct, I think they should. It's scary right now what's happening," said Chiara Alcivar.
"We have to keep the good faith, we cannot stop from moving," said another passenger, Philip Bustani.
Homeland Security Secretary Jeh Johnson stressed there is no "specific, credible intelligence" pointing to a similar plot in America, but he said the Transportation Security Administration would deploy additional security at major airports and rail stations in different cities. Officials also reviewed additional security measures for travelers from Belgium, among more than three dozen countries whose citizens generally don't need a visa to enter the U.S.
The attacks in the Belgian capital underscored the growing threat posed by the Islamic State group on both sides of the Atlantic. The bombs in Brussels' airport and subway locked down the European Union's capital just a few months after attacks shocked Paris and San Bernardino, California.
"We will do whatever is necessary to support our friend and ally Belgium in bringing to justice those who are responsible," Obama declared in Havana, where he was closing his historic, three-day visit. The attack immediately overshadowed events on the island, with President Barack Obama vowed to help Belgium track down those responsible for the deadly explosions.addressing the tragedy at the top of a keynote speech to the Cuban people and again at an exhibition baseball game.
"The world must unite," Obama said after offering his condolences in a telephone call with Belgian Prime Minister Charles Michel. "We can and we will defeat those who threaten the safety and security of people all around the world."
Following the attacks, U.S. European Command announced new prohibitions on unofficial military and Defense Department employee travel to Brussels "until further notice." Official travel to the NATO hub in the city now requires approval.
Secretary of State John Kerry, accompanying Obama in Cuba, said in a statement the U.S. was working to determine the status of all Americans in Brussels. The embassy there issued a statement telling U.S. citizens to stay where they are and "take the appropriate steps to bolster your personal security."
In the United States, the Homeland Security Department said it could further enhance security measures "as appropriate, to protect the American people." It urged Americans to report any suspicious activity to law enforcement authorities.
RELATED: Here's how New York City area is beefing up security in response to Brussels terror attacks
(The Associated Press contributed to this report.)
State Department issues travel alert for Americans going to Europe
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