Chinese fishing boats are leaving the tense inter-Korean border in the Yellow Sea after North Korea's threat of military action, South Korea's Yonhap news agency reported Friday.

"Chinese fishing boats operating near the Northern Limit Line (NLL) began withdrawing yesterday," it quoted a defence source as saying, adding military authorities are trying to find out whether North Korea asked them to do so.

The NLL, which North Korea refuses to recognise, marks the maritime border off the west coast of the peninsula.

More than 280 Chinese vessels were fishing near the NLL for crab earlier this week but the number has fallen to about 140, according to the source.

The North warned Wednesday of possible military action after South Korea announced it is joining the Proliferation Security Initiative (PSI), a US-led campaign to stop the trade in weapons of mass destruction.

The North also declared it is no longer bound by the armistice that ended the 1950-53 Korean war.

The South announced its decision to join PSI one day after the North conducted its second nuclear test.

The area around the NLL was the scene of bloody naval clashes in 1999 and 2002.