The South Korean navy fired warning shots Wednesday at a North Korean vessel that crossed a disputed maritime border. The incident comes amid tensions over the North's nuclear ambitions.

A statement from South Korea's Joint Chiefs of Staff says a North Korean patrol boat sailed more than two kilometers south of the two nations' sea border. The boat remained in South Korean waters for 14 minutes, then retreated after a South Korean warship approached and fired warning shots.

A lethal naval battle took place in June in the same area after a ship from the North crossed the boundary and fired on a South Korean vessel, killing six sailors. The South returned fire and killed an estimated 13 North Koreans.

The maritime border has been the site of naval clashes between the two Koreas for decades. North Korea does not recognize the Northern Limit Line roughly halfway between the two. The line was drawn by the U.S.-led United Nations command after the Korean War ended with no peace treaty in 1953.

Wednesday's incident comes amid growing international concerns over North Korea's nuclear weapons program. Pyongyang told an American envoy last month that it had a secret uranium enrichment project, in violation of several international accords. Enriched uranium can be used to create nuclear bombs.

In response to the admission, the United States, the European Union, South Korea and Japan decided last week to suspend fuel oil deliveries to the North from December.

The shipments were part of 1994 pact in which North Korea agreed to freeze its nuclear program in exchange for fuel aid. In return, the U.S.-led Korean Peninsula Energy Development Organization, or KEDO, promised to send North Korea fuel oil and build two light-water reactors to supply power to the impoverished communist state.

Japanese government spokesman Yasuo Fukuda said Wednesday that Tokyo has not changed its stance on the reactors and remains committed to its role in building them.

His comments come after a U.S. official said Tuesday that the future of the reactors is open for review. KEDO board members will meet on December 11 and could make a decision on the reactors then. So far, North Korea has not reacted to the suspension of the fuel shipments, but it had warned previously it would view such a move as a hostile act.