New York and Washington stepped up security in the wake of the attacks in Brussels on Tuesday, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said.

New York and Washington stepped up security in the wake of the attacks in Brussels on Tuesday, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said.

The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute.

"These teams have been deployed to crowded areas and transit locations around the city out of an abundance of caution to provide police presence and public reassurance," the NYPD said in a statement.

Police armed with machine guns patrolled Times Square, one of the most popular tourist destinations in New York in the heart of the theater district, an AFP photographer said.

Governor Andrew Cuomo ordered increased security at New York's flagship airports, John F. Kennedy International and LaGuardia, as well as bridges, tunnels and commuter rail systems.

Around 400 National Guard personnel were on duty in the area, Cuomo's office said.

State police were also assigned to New York's Penn and Grand Central Stations, the World Trade Center, the city's bus terminal and the George Washington Bridge, which connects Manhattan to New Jersey.

In the US capital, metro transit police said they were ordering extra K9 sweeps and patrols as a "precaution," adding that there was no known, specific or credible threat to Washington D.C.

Armed police were also seen out in force at Dulles Airport in Washington.

Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO.