South Korea’s coast guard has for the first time opened fire on Chinese fishing trawlers, which are straying into Seoul’s territorial waters amid growing Chinese demand for fresh seafood.

Tuesday’s incident comes less than a month after a warning from Seoul that it would use a “more aggressive” firearms policy in encounters with illegal fishing vessels. This was prompted by the sinking of a 4.5-tonne Korean coast guard ship that had been rammed while trying to intercept a trawler.

The coast guard seized two trawlers after firing about 700 M60 machine gun rounds at a fleet of 30 fishing ships that South Korean authorities say had “rushed” its vessels. No one was killed or injured in the incident, which coast guard official Kim Jung-shik said was sparked by the threatening behaviour of the fishermen.

“Initially, we sent warnings but they kept approaching us, making the situation serious,” Mr Kim told Yonhap, South Korea’s state news agency. “I thought that if we tolerated their resistance further, the coast guard men would be in jeopardy. So we used the machine gun.”

The incident in the Yellow Sea west of the port of Incheon is likely to be another blow to relations between Seoul and Beijing, which have been strained in recent months following a decision to place a US anti-ballistic missile system in South Korea.

In response to the incident, China’s ministry of foreign affairs expressed “strong dissatisfaction” in a statement on Wednesday evening.

“We have lodged multiple solemn representations to Korea on its coast guards’ violent enforcement techniques.”

Authorities in Seoul say the Terminal High Altitude Air Defence platform, or Thaad, is aimed at preventing strikes from North Korea, which recently conducted its fifth nuclear test. Beijing, however, views the deployment as a threat to its own nuclear deterrent and an attempt to further US surveillance of China.

The clash with the South Korean coast guard marks at least the third time this year that Chinese fishermen have been attacked by foreign law enforcement vessels.

In March Argentina sank a Chinese fishing boat that it alleged had been fishing illegally in its waters, while in June Indonesia injured one sailor after opening fire in a similar incident.