South Korean coastguard vessels have, for the first time, fired machine guns against Chinese boats illegally fishing in Korean waters, an official said Wednesday.

No casualties were reported from the incident on Tuesday, the first of its kind since the coastguard announced last month that it would pursue a "more aggressive" firearms policy with Chinese trawlers.

Disputes over illegal fishing have dogged relations between South Korea and China for years, and there have been numerous clashes between the coastguard and Chinese crew members.

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 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 urging 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.