A South Korean navy ship sank after colliding with a larger fishing boat, leaving one sailor dead and two missing, the military said Thursday.

The 150-ton navy vessel was returning to its base following a routine patrol mission Wednesday when it collided with a 270-ton South Korean fishing boat in waters northwest of the southern resort island of Jeju, a spokesman at the Joint Chiefs of Staff said. The cause of the collision wasn't immediately known.

Twenty-eight navy sailors were rescued, but one of them died while being treated at a hospital on the island. Two others were missing, the spokesman said on condition of anonymity citing department policy.

Some of the injured sailors were being treated at a hospital, while others were to be sent back to their base, he said. Navy vessels and helicopters were searching for the missing crew.

Damage to the fishing boat wasn't severe, the spokesman said. It was not immediately known how many crew were aboard the fishing boat, but another Joint Chiefs of Staff spokesman said his office has not received any reports that any of them were injured.

The navy plans to conduct salvage operations on the sunken vessel.

The fishing boat sailed to Jeju Island after the collision, according to the first spokesman. The island, about an hour's flight from Seoul, is a popular honeymoon resort for South Koreans.

South Korea's military is on its highest alert ahead of a two-day gathering of leaders from the G20 advanced and developing economies in Seoul.