South Korea's military vowed Tuesday to protect the disputed Yellow Sea border with North Korea after the communist state threatened to open fire in waters controlled by Seoul.

"We are maintaining the usual watertight vigilance against any possible North Korean provocations, to safeguard the NLL maritime border," a Joint Chiefs of Staff spokesman told AFP.

The North refuses to accept the border known as the Northern Limit Line drawn by United Nations forces after the 1950-53 war. It demands the border be drawn further south and said Monday it would recognise only its own line.

The communist state's navy declared waters near its own self-declared border as a "peacetime naval firing zone" for coastal and island artillery units, and called on southern boats to avoid the zone.

"We want them to stop making the far-fetched argument," the Joint Chiefs spokesman said, adding there were no unusual movements Tuesday in the area.

Tensions have remained high in the Yellow Sea since a brief but intense gunfight on November 10 left a North Korean patrol boat in flames. There were deadly naval gunbattles there in 1999 and 2002.

The South's navy expressed regret at its neighbour's announcement and pledged it would "sternly" handle any provocations.

"We express grave concern as the threat … raises unnecessary tension between the Koreas," a statement said late Monday.

"Our military is fully prepared to protect the NLL and it will sternly respond to North Korea's provocations."

The renewed tensions at sea come despite an overall improvement in relations.

Last Friday South Korea shipped swine flu medicine worth 15 million dollars to its impoverished neighbour, the first direct government aid for nearly two years.

On Monday Seoul said it may fund assistance projects by several international organisations for North Korean babies and children.

The two nations were Tuesday set to complete connection of Seoul-funded military communication lines across the overland border.

The South's unification ministry, which handles cross-border relations, said the lines would be tested before being formally opened next week.

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