(AP) --Authorities stepped up security throughout New York City on Tuesday after explosions in the Belgian capital of Brussels killed at least two dozen people in the airport and the subway.

"The New York City Police Department is closely monitoring the situation in Belgium and is in close contact with our international partners and with the FBI," the NYPD said in a statement.

The agency said there was "no known indication that the attack has any nexus to New York City," but it was deploying additional counterterrorism units to crowded areas and transit locations across the city "out of an abundance of caution."

The Port Authority Police Department also increased security at its three area airports -- John F. Kennedy, LaGuardia and Newark Liberty -- and bridges, tunnels and bus terminal. It placed high-visibility anti-terrorist patrols throughout the Port Authority Trans-Hudson system and the World Trade Center site. Additional bag checks also were being conducted at PATH stations as a precaution.

The explosions in Belgium on Tuesday morning prompted a lockdown of the capital and heightened security across Europe. Belgium raised its terror alert to the highest level, diverting planes and trains.

The Brussels prosecutor's office called the explosions terror attacks. They came just days after the main suspect in the November attacks in Paris was arrested in Brussels. There was no immediate claim of responsibility for Tuesday's attacks.