South Korea's coastguard promised Monday to release 21 Chinese fishermen who were arrested last week for illegal fishing in the Yellow Sea.

The Chinese aboard two boats were captured on Saturday near the Gageo-do island in an operation involving a helicopter and coastguard ships.

They are in custody in Mokpo, a port city about 290 kilometres (174 miles) southwest of Seoul.

"The Chinese brandished clubs and shovels but our team using tear gas and rubber bullets quickly overpowered them," a coastguard spokeswoman in Mokpo told AFP.

One crew member was hospitalised after a rubber bullet hit his face, she said.

"They will be released soon if they pay a fine for fishing illegally in our territorial waters," she said.

China's foreign ministry urged South Korea "to conduct law enforcement in a civilised way, avoid violence and properly ensure the legitimate rights" of the fishermen.

"We will follow developments closely," spokeswoman Jiang Yu said in Beijing.

Illegal fishing by Chinese vessels is common in South Korean waters. In 2008 a South Korean officer drowned while trying to inspect a Chinese boat, and 10 officers have been injured this year.

In December 2009, a Chinese trawler capsized after ramming into a coastguard ship, leaving one crew member dead and another missing. Four South Korean officers were injured.