A seven-ton electromagnet is to be used to salvage the missing people and vehicles after a bridge collapsed in south China's Guangdong Province, local maritime affairs department said on Sunday.

The 9,000-watt magnet, which has an attraction force of 30 tons, arrived at the scene early Sunday evening. Initially it might be used to salvage an object located 900 meters downstream from the site of bridge collapse.

Experts suspected that the object was the debris of a vehicle about 20 meters underwater. But it was not decided whether the operation would be launched Sunday night, because the water was flowing fast.

Earlier salvage efforts by frogmen failed because the water, which is 33 to 36 meters deep, was beyond their working conditions.

Rescuers then tried to hook the vehicles and pull them out with cable, but this didn't work either.

Video images from traffic monitors showed four vehicles carrying seven people, and two bridge workers were on the bridge when it collapsed after being hit by a sand barge.

Six crew members, including their captain, have been detained by maritime police.

The 1,600-meter-long bridge connects Jiujiang Township in Nanhai district of Foshan to neighbouring Heshan city.

Sections of the collapsed bridge fell onto the bow of the boat, partially submerging it. All 10 crew members were rescued, and two were released from hospital after being treated.