Major cities in the United States ramped up security Tuesday following the attacks in Brussels, as police armed with machine guns patroled New York's Times Square and National Guard troops deployed to airports.
Counterterrorism police were also out in force and authorities -- while stressing there was no credible threat to US soil -- ordered bolstered security measures as millions of Americans embarked on their morning commute.
In New York, National Guard reinforcements were sent to its two international airports, police patrols doubled during the rush hour and security officers boosted at rail and bus stations, officials said.
"There is no specific and credible threat," said New York Mayor Bill de Blasio. "But we are in a high state of vigilance and readiness."
Police toting machine guns kept a wary eye over Times Square, one of the most popular tourist destinations in New York in the heart of the theater district, an AFP photographer said. Police said the increased presence would last several days.
Tuesday marked the first day of a new 55-person dedicated unit at the Times Square subway station, used by 500,000 passengers a day, designed to combat crime, increase visibility and counter-terrorism capability, police said.
Around 400 National Guard personnel were on duty, said New York Governor Andrew Cuomo, who ordered bolstered security at airports, bridges, tunnels and commuter rail systems.
State police were 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 Washington, metro transit police said they were ordering extra K9 sweeps and patrols as a "precaution," despite there being no known, specific or credible threat to the US capital.
Armed police were also seen out in force at Dulles Airport, just outside Washington.
In Los Angeles, security was also beefed up at the airport, the third-busiest in the country, a spokesman said.
Police said security was also being strengthened at Hollywood landmarks as well as key events taking place throughout the city, including sports matches.
About 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city. The Islamic State group claimed responsibility.