Senior coastguard official Kim Jung-shik said the order to fire came during a stand-off with some 30 Chinese fishing boats illegally operating near the South’s Yellow Sea border with North Korea.

“They tried to ram our ships although we repeatedly warned them,” Kim told the Yonhap news agency.

“I thought our officers would be in danger if I allowed any more resistance so we ended up using the crew service weapon,” he was quoted as saying.

Initial machine gun bursts were fired into the air, but the crew were later ordered to fire on the bows of the Chinese boats that were sailing directly at the coastguard vessels.

Two Chinese trawlers were seized in the clash.

China’s foreign ministry said it was “strongly dissatisfied” at the action and urged Seoul to “discipline” its coastguard.

“Using destructive weapons can easily hurt fishermen and we urge the ROK (South Korea) side to... avoid using any excessive or extreme tools in their law enforcement activities,” spokeswoman Hua Chunying said.

Seoul has been asking Beijing to take a tougher stand on its vessels that have entered the South’s waters in increasing numbers to satisfy growing demand at home for fresh seafood.

Small wooden Chinese ships were once tolerated in an area where the top priority has always been guarding against potential incursions from North Korea.

But in recent years, the small boats have given way to larger steel trawlers which engage in bottom trawling - dragging a large weighted net across the seabed that sweeps up everything in its path.