Washington, May 3 (IANS) The bombing attempt in New York's Times Square was an "extremely serious situation" and the US authorities are investigating all possibilities, the White House said Sunday.

Robert Gibbs, press secretary for the White House, refused to characterise who might be involved or what those motives might be during a press brief onboard Air Force One, Xinhua reported.

Gibbs made the remarks when he was asked about the assertion by US Homeland Security Secretary Janet Napolitano that this incident is terrorism.

He said it was an "extremely serious situation" and the authorities, including the New York Police Department and FBI, are investigating all possibilities.

A T-shirt peddler alerted local authorities of a suspicious, smoking sports utility vehicle on Times Square in downtown New York Saturday evening. Bomb squad officers later discovered propane gas tanks, fireworks and timing devices in the vehicle.