LUCKNOW, India -- A highway overpass that was under construction in north India collapsed Tuesday, killing at least 16 people when an immense concrete slab slammed down onto the crowded road below, disaster officials said. More people were feared trapped after the collapse occurred in the city of Varanasi, crushing at least four cars, a motorcycle and a bus. It was not clear how many people were in the vehicles.

The slab appeared to be at least 50 feet long and 6 feet wide.

"There was sudden rumble and within seconds we saw" the vehicles crushed, local resident Ramesh Kumar Singh said in a telelphone interview. "It took at least a minute for the people around to realize what exactly had happened."

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Most of the people killed were traveling in vehicles, said senior police officer P.V. Ramasastry.

By nightfall, rescuers had pulled three people alive from the rubble, according to a statement from the National Disaster Response Force.

The state's top official ordered a probe into the collapse of the overpass that was being built. He also announced a 500,000 rupee ($7,200) payment to families of those who died.

India has a long history of construction accidents caused by poor materials and inadequately trained workers.