North Korea on Saturday accused the South of instigating clashes around the disputed sea border to rally support for the ruling conservative party ahead of December's presidential election.

A spokesman for the North's powerful National Defence Commission claimed the South's "warmongers" were "infiltrating warships deep into the territorial waters... almost every day".

He said the South's warships had recently opened fire without prior warnings at fishing boats operating near the disputed maritime border. The North has claimed the fishing boats were from China.

The South is now seeking to justify the infiltrations and shooting incident as measures for defending the "northern limit line", he said in a statement.

"And Park Geun-Hye and other ambitious politicians are using them for stepping up the presidential campaign," the spokesman said.

The North never recognized the border line, unilaterally drawn by the US-led United Nations forces at the end of the Korean War to prevent naval clashes between the two Koreas.

Weeks of repeated incursions by North Korean fishing vessels over the disputed Yellow Sea border culminated in the South Korean navy firing warning shots last week to push the boats back.

Pyongyang earlier denounced the "shooting rampage" and threatened to respond with a "powerful strike of the (North's) front units which know of no limit".

The statement Saturday came after Seoul this week threatened a strong military response to any provocative act by the communist state aimed at influencing the December 19 vote, in which Park, the ruling conservative party candidate, hopes to become South Korea's first woman president.