A South Korean naval vessel fired warning shots Friday after a North Korean patrol boat and fishing boat crossed the countries' disputed maritime border, Seoul's defence ministry said.

The incident coincided with a prolonged push by North Korea for talks to reduce military tensions that escalated sharply after Pyongyang conducted its fourth nuclear test in January.

The two North Korean vessels crossed the Yellow Sea border around 7:30am (2230 GMT Thursday) but swiftly retreated after a South Korean navy patrol boat fired five warning shots, a ministry official said.

The de-facto maritime boundary between the two Koreas -- the Northern Limit Line -- is not recognised by Pyongyang, which argues it was unilaterally drawn by US-led United Nations forces after the 1950-53 Korean War.

Both sides complain of frequent incursions by the other and there were limited naval clashes in 1999, 2002 and 2009.

Incidents like Friday's intrusion are quite common and rarely escalate into anything more serious.

In recent weeks, Pyongyang has been urging Seoul to accept leader Kim Jong-Un's proposal for military talks aimed at easing cross-border tensions.

Kim had offered the military dialogue during a marathon speech to the recently concluded congress of North Korea's ruling Workers' Party -- the first event of its kind for more than 35 years.

Seoul has rejected the proposal, saying it lacks sincerity given Kim's vow to push ahead with the North's nuclear weapons and ballistic missile programmes.

South Korea says it will only engage in substantive dialogue if the North takes a tangible step towards denuclearisation.

The North has repeatedly said its nuclear arsenal is not up for negotiation.