SEOUL (AFP) – A South Korean patrol ship sank off the southern island of Jeju after a collision with a fishing boat, leaving one sailor dead and two others missing, navy officials said Thursday.

The 150-ton naval ship with 30 sailors aboard had been on a routine mission when it collided with a trawler late Wednesday, a navy spokesman said.

Another patrol ship rescued 28 sailors but one died soon after being taken to a hospital, he said.

"Several ships, helped by helicopters, have been involved in an operation to search for two missing sailors," he said.

Navy officials said the sinking might have been caused by the influx of water after the fishing boat collided with the patrol ship.

The 270-ton fishing boat suffered minor damage.

South Korea's military has been put on top security alert to guard this week's G20 summit against any disruptions by North Korea or international terrorists, with naval forces monitoring sea vessels.

Cross-border tensions have been high since a South Korean warship sank near the maritime border with North Korea in March with the loss of 46 sailors.

Seoul and Washington blamed Pyongyang and an international inquiry found that a North Korean torpedo had sunk the 1,200-tonne corvette.

Pyongyang has consistently denied the charge.