The KT-1 trainer jets crashed near Sacheon town, about 300 kilometres south of the capital Seoul, according to military authorities.

"Two KT-1 trainer jets at the Sacheon Air Base collided in mid-air while training and crashed," the defence ministry said in a statement.

Three pilots were killed in the accident, and one remains missing, it added.

The military will "check the exact situation of damage and investigate the cause of the accident," the statement added.

Over 30 firefighters and emergency responders were dispatched to the crash site, the Yonhap news agency reported.

Friday's crash is the latest incident involving South Korea's air force jets this year.

In January, an air force pilot was killed in a F-5 fighter jet crash.

That incident took place shortly after the country grounded its entire fleet of advanced F-35 fighter jets, after one of them was forced to make a dramatic emergency landing after a major systems malfunction.

The Korean Peninsula remains in a technical state of war, as the armistice that ended the conflict in 1953 has yet to be replaced with a peace treaty.