SEOUL: 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 defense 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 7:30 a.m. (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 recognized 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.