"A North Korean patrol boat briefly intruded into our waters before returning to the North following warning shots from our side," a spokesman for the Joint Chiefs of Staff told AFP.

The incident occurred at 2:40 pm (0540 GMT) in the disputed waters around Baekryeong island in the Yellow Sea, where a deadly inter-Korean naval skirmish erupted in June.

The North Korean boat crossed the maritime border, known as the Northern Limit Line (NLL), at a point some 5.6 kilometres (3.5 miles) from the island, prompting five South Korean navy vessels to move to intercept it.

One of the South Korean boats fired two warning shots from its 76-millimeter cannon before the North Korean vessel fled to the north. The North Korean ship did not fire back.

"It is believed that the North Korean boat had crossed the NLL as it was chasing Chinese fishing boats," the Joint Chiefs of Staff said in a statement.

At the time of the incursion there were reportedly around 20 Chinese fishing boats engaged in illegal fishing in the rich fishing ground.

It was the second incursion by a North Korean boat into South Korean waters in four days, following a similar incident in the same area on Saturday when a North Korean patrol boat crossed the NLL in pursuit of Chinese fishing boats.

The North has never acknowledged the NLL, which was unilaterally drawn by the US-led United Nations during the Korean War of 1950-53 to prevent ships from crossing to the North from the South.

The shooting marked the first use of naval firepower in the disputed waters since a deadly skirmish erupted in the disputed waters of the Yellow Sea in June which left five South Korean naval personnel dead. It also claimed an unknown number of casualties on the North Korean side.

North Korea surprised the world by quickly apologizing over the skirmish, resulting in the resumption of inter-Korean talks.

The timing of the back-to-back incursions has alarmed South Korean authorities since they come at a time when ties between North Korea and the United States are at the lowest point in years over the North's suspected nuclear weapons programme.

But analysts here downplayed the incursions as accidental.

"It is hard to believe that the North did it intentionally," political science professor Koh Yoo-Hwan told AFP. "There are few gains for the North to be had by increasing tension intentionally," he said.

The United States and its allies last week decided to cut off fuel oil shipment to energy-starved North Korea which they accused of violating international nuclear safeguard accords.

Last month, US special envoy James Kelly said Pyongyang had admitted to running a secret weapons program using enriched plutonium in violation of the 1994 Agreed Framework.

Under the accord, Pyongyang promised to halt nuclear weapons programs in return for the construction by a US-led consortium of two light-water reactors and the delivery of 500,000 tonnes of heavy fuel oil a year.

The North has neither officially responded to the recent halt to the heavy oil supply nor publicly acknowledged its enriching of nuclear fuel.

SPACE.WIRE