A North Korean naval patrol boat briefly crossed the disputed Yellow Sea border with South Korea on Thursday while tracking Chinese fishing vessels, military officials in Seoul said.

The apparently accidental incursion, which the officials said lasted only seven minutes, came amid heightened tensions on the Korean peninsula ahead of the South's presidential election in December.

The patrol boat sailed about 36 metres (120 feet) into the southern side of the sea boundary but swiftly retreated after a warning radioed by South Korean naval ships, the South's Joint Chiefs of Staff (JCS) said.

"We believe the North Korean boat accidentally crossed the Northern Limit Line (NLL) while trying to chase away Chinese fishing vessels," a JCS spokesman told AFP.

About 70 Chinese vessels were fishing near the border, he said.

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

It was a scene of bloody battles in 1999, 2002 and 2009.

In September, South Korean navy patrol ships fired warning shots at North Korean fishing boats that crossed the border.

The two Koreas remain technically at war since the Korean conflict was concluded with a ceasefire rather than a peace treaty, and small border incidents in the past have been known to escalate swiftly.

Thursday's incursion came as South Korea kicked off an annual, large-scale military exercise aimed at countering threats from North Korea.

The week-long military manoeuvres will involve 240,000 army, navy, air force and marine corps personnel, along with police officers.

Pyongyang was enraged by a US-South Korea agreement announced earlier this month to almost triple the range of the South's missile systems to 800 kilometres (500 miles) to cover the whole of North Korea.