SEOUL >> A fire at a hospital annex housing elderly patients in the southwestern county of Jangseong killed 21 people early Wednesday, officials said.

Twenty patients and a nurse were killed and seven others were injured, officials with the Jangseong Fire Department said, speaking on condition of anonymity because of office rules.

The victims died after suffocating on poisonous gas, officials said.

There were 33 patients and a nurse on duty when the fire broke out on the second floor of an annex of Hyosarang Hospital. The fire was put out after about six minutes, the officials said. The cause was not immediately known.

The facility specializes in elderly patients who suffer from dementia and palsy, officials sasid.

The fire comes as South Korean officials are still searching for missing passengers from a ferry sinking last month that killed or left missing more than 300 people. South Korea has also had two subway accidents in recent weeks.