About 40 passengers and crew aboard a Korean Air jet were injured on Thursday when the plane hit severe turbulence and took a rapid dive, the airline said.

Korean Air Flight 630, with 257 people aboard, hit the turbulence about 40 minutes after departing Indonesia's resort island of Bali bound for South Korea.

A 35-year-old man suffered a severe head wound and 11 crew members were injured, it said. All were treated at a hospital near the South Korean capital Seoul and released, except for the man with a serious head injury.

The airline said the Boeing 747 hit a patch of "clear air" turbulence, which does not appear on radar.

It added that radar picks up most patches of air turbulence but that it has about 10 to 20 incidents of clear air turbulence a year, mostly minor.