At least 29 people have been injured in an explosion in a crowded district of New York City.

The cause of the blast late on Saturday in Chelsea remains unclear. Mayor Bill de Blasio described it as "intentional" but said that there were no known links to terror.

The force of the blast blew out windows and could be heard several blocks away.

Another device, reportedly a pressure cooker rigged with wires, was later found in the same district.

None of the injuries is life-threatening but one is serious, New York's fire commissioner said.

Most of those hurt suffered cuts and other minor injuries from glass and other debris, CNN reported.

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