South Korea’s president said Saturday his country won’t give in to North Korea’s nuclear and missile threats, while Pyongyang accused Seoul of sending patrol boats into its territorial waters — the scene of past bloody naval clashes.

The North’s official Korean Central News Agency alleged the patrol boats were sailing into North Korean waters daily around the rivals’ disputed western sea border. The Korean-language report warned that aggressors would be dealt “merciless punishment that will be beyond imagination.”

The claim was rejected by Seoul, which two days ago alleged one of the North’s patrol boats violated its sea border in the same area. The boat turned back without incident after a 50-minute standoff with the South’s naval ships, the South Korean military said.

The disputed waters — where deadly clashes occurred in 1999 and 2002 — are a potential flash point for the rivals. Many fear a minor dispute could quickly escalate into a major confrontation, especially with tensions soaring after the North’s May 25 nuclear blast and recent missile tests.

Earlier Saturday, South Korean President Lee Myung-bak delivered a stern warning to the North in a nationally televised Memorial Day speech honoring the country’s war dead at Seoul’s National Cemetery.

“I would like to make it clear that there will be no compromise against things that threaten our people and security,” Lee said.

Lee’s words echoed those of U.S. officials, who have also said the North’s former tactics of using military threats to win much-needed food and energy aid would no longer work. Washington is considering punishing North Korea with its own financial sanctions, apart from whatever the U.N. might decide to adopt.

At the U.N., lengthy closed-door negotiations about sanctions appeared to be close to an end. The measure was being worked out by five veto-wielding Security Council nations — the U.S., China, Russia, Britain and France — along with Japan and South Korea.