SEOUL - North Korea's military warned the South that it was ready for battle over a disputed sea border, issuing a new threat on Friday that raised tension as U.S. President Barack Obama started a major tour of Asia.

Obama said in an interview that North Korea's nuclear and missile programs posed a grave threat to Asian security and Pyongyang should return to dormant disarmament talks.

The two Koreas had their first naval firefight in seven years on Tuesday along a disputed sea border off their west coast that left a South Korean vessel pockmarked with about a dozen bullet holes and a North Korean patrol ship ablaze as it retreated home.

The North's military said in a message carried by the state's KCNA news agency that it did not regard as valid the naval border set unilaterally by U.S.-led U.N. forces at the end of the 1950-53 Korean War, which was halted by a cease fire.

Its military pledged to take merciless military measures to defend what it saw as the correct line.

A day earlier, North Korea said the South would pay an expensive price for firing at its retreating patrol boat.

The sea border has been the site of two deadly naval battles between the Koreas in the past decade. There were no reported causalities from the gun fight on Tuesday.

The two Koreas are technically still at war.

Obama started his Asian tour in Japan. His administration this week said it would dispatch its first official envoy to Pyongyang to bring new life to stalled six-country disarmament-for-aid talks with the impoverished North.

We are open to a bilateral meeting as part of the six-party process if that will lead to an expeditious resumption of the denuclearization negotiations, Obama said in an interview with the South's Yonhap news agency.

North Korea for nearly a year has boycotted the six-way talks aimed at having it scrap its nuclear program in exchange for aid to rebuild its broken economy and a better diplomatic standing that could help it receive international finance.

North Korea's attempt to acquire weapons of mass destruction and the means to deliver them is destabilizing and represents a threat to peace and security, Obama said.

Analysts said Washington would not have signed off on the visit of its envoy Stephen Bosworth unless it had assurances Pyongyang would return to disarmament talks.

(Reporting by Jon Herskovitz and Jack Kim; Editing by Jeremy Laurence)

Japan Barack obama South Korea