A five-storey building being demolished in southern South Korea collapsed, sending debris falling on a bus and killing four people on board, officials said.

Concrete from the collapsed building in the southern city of Gwangju fell on the bus when it stopped on a nearby street, the National Fire Agency said.

Emergency officers dispatched to the site rescued eight people from the bus, all seriously injured, before discovering the four dead bodies, an agency official said on condition of anonymity citing department rules.

He said emergency officers were searching for any other people who might be trapped under the debris.

The official said all construction workers at the building site were evacuated before its collapse.

An earlier fire agency statement said the debris fell on two passenger vehicles.

But agency officials later corrected that analysis after watching security video footage.

Another video from the scene showed dozens of rescue workers equipped with stretchers and crowbars searching for survivors while excavators hacked at a huge mountain of crumbled concrete and bent steel beams that spilled over a motorway.