New York's World Trade Center was lit in solidarity with Belgium after around 35 people were killed in bombings in Brussels.
New York Governor Andrew Cuomo posted a picture of the 408-foot (124-meter) spire colored black, yellow and red on twitter and said it was lit "in solemn solidarity with the people of Belgium."
But the tribute from the tallest building in the US, built on the site of the 9/11 attacks in New York, sparked confusion.
A series of photographs published by AFP and other media showed the colors to be blue, white and red -- those of the French flag that lit up the Trade Center after the November attacks in Paris.
An AFP photographer said the colors on the World Trade Center appeared correct to the naked eye, but did not translate so accurately on film, shot against the dark night-time sky.
Uptown, New York's iconic Empire State Building remained dark, foregoing its traditional white, in sympathy for the lives lost.
Dozens of Belgian expatriates gathered in Union Square in Manhattan after night fell, bringing flowers, candles and flags to express solidarity with the victims and survivors of the attacks.
"It's important to be here," said Renaud Vanlangendonck, 33, a former teacher carrying his five-month-old daughter. "We saw it in New York, Paris, Istanbul... And now it's our country, it's horrible."
There was a visible police presence in Union Square as law enforcement bolstered security across America's largest city.
The FBI and New York police were dispatching detectives as early as Tuesday night to investigate the Brussels attacks with at least three US citizens -- Mormon missionaries -- among the casualties.
John Miller, deputy police commissioner for intelligence and counterterrorism, made the remarks as the US government, state and city authorities stepped up security at airports and transit hubs.
