Crane ships and frogmen began sifting through the wreckage of a collapsed bridge in Indonesian Borneo on Tuesday, as more bodies were fished out of the river, raising the death toll from the weekend disaster to 18.

Authorities have still given no reason for the cause of Saturday's tragedy, when a 720-metre-long bridge -- built to resemble San Francisco's Golden Gate -- collapsed over the Mahakam river and dumped dozens of vehicles into the water.

But police said they had talked to witnesses and would also question the construction and maintenance companies.

"We recovered this morning five bodies which were floating in the water," East Kalimantan province police chief Bambang Widaryatmo told reporters, adding that dozens of people were still believed missing.

The disaster also injured 39 people, authorities said.

Eighteen professional divers, including navy frogmen with echo sound equipment, began the underwater search for the first time Tuesday, after being hampered for days by strong currents that had restricted visibility.

"The divers started this morning to go into waters 50-metre deep to recover bodies trapped in vehicles and the wreckage," National Disaster Management Agency spokesman Sutopo Purwo Nugroho said.

An AFP correspondent saw frogmen diving into the water, as nine tugboats and three crane ships removed wreckage from the surface.

Widaryatmo said the boats were removing the bridge debris to help the search for bodies.

Witnesses to the accident reportedly heard a loud crashing sound as the bridge buckled, sending a public bus, cars and motorcycles plunging into the broad river in Kutai Kartanegara district.

Survivors desperately swam to the shore, screaming in panic, while others were trapped underwater beneath the debris.

The cause of the collapse was not immediately clear but Nugroho said on Sunday that a steel support cable for the bridge, finished in 2002, snapped as workers were repairing it.

The Jakarta Post on Monday quoted Public Works Minister Djoko Kirmanto as saying the bridge had been weakened after being struck by boats several times.

"A pillar almost collapsed last year because it was hit by a cargo barge that carried coal," Kirmanto told the daily.

Indonesian President Susilo Bambang Yudhoyono has ordered an investigation into the cause of the accident.

Indonesia is setting a blistering pace of growth, expected to top six percent this year, but investors complain infrastructure is hopelessly inadequate and that the nation is mired in corruption and red tape.