SEOUL - South Korea's navy bolstered its defences near the tense sea border with North Korea Tuesday, sending in a high-speed patrol boat armed with guided missiles and vowing to "punish" any attacking forces.

Compared with North Korean vessels, the South Korean ship "is armed with overwhelming fire power," a naval spokesman told reporters.

The South Korean navy will "punish immediately" any North Korean forces attempting provocative acts in the area, he said.

The North is reported to have stepped up naval drills near the western maritime border -- the site of deadly skirmishes between the two Koreas in 1999 and 2002 -- after threatening to attack the South.

Pyongyang has warned of "self-defence measures" in response to any tougher international sanctions for last week's nuclear test, and US and South Korean officials say it appears to be preparing to test-fire a long-range missile.

South Korea's Yonhap news agency reported Tuesday that the North also seems to be preparing to test-fire a medium-range missile from its southeast coast.

It said the missile was believed to be a Rodong with a range of 1,300 kilometres (about 800 miles) or a new medium-range missile with a range of 3,000 kilometres.

Tensions have been running high since Kim Jong-Il's regime tested a nuclear bomb for the second time on May 25. It then launched a series of short-range missiles and renounced the truce that ended the Korean war in 1953.

South Korean and Southeast Asian leaders Tuesday condemned the North's nuclear test as they wrapped up a summit on the southern resort island of Jeju.

President Lee Myung-Bak and the 10 ASEAN leaders in a statement said the test and recent missile launches were "clear violations" of UN Security Council resolutions and a multi-nation nuclear disarmament pact.

Analysts say Kim, who is believed to have suffered a stroke in August last year, may be trying to bolster his authority to prepare for an eventual succession.

The North Korean leader is believed to have nominated his third and youngest son, Jong-un, to succeed him, a South Korean lawmaker briefed by intelligence officials said Tuesday.

According to reports, North Koreans are being taught new songs aimed at instilling loyalty to the next leader, addressing Jong-Un as "General Kim."

Jong-Un was born to the leader's third wife, Ko Yong-Hi, who reportedly died of breast cancer in 2004. He was educated at an international school in Switzerland.

Kim's eldest son -- Jong-Nam, 37, who was born to a different mother -- apparently spoiled his prospects of becoming leader after being deported from Japan in 2001 for trying to enter the country with a forged passport.

Reports of the nomination come with South Korean and US forces on the peninsula on heightened alert after the North threatened an attack when Seoul joined a US-led initiative to stop the spread of weapons of mass destruction.

North Korea's military has reportedly been using high-speed boats for landing exercises near the western border, which Pyongyang wants to be drawn further south.

Washington warned North Korea Monday not to fire a long-range missile, saying it would further worsen tensions.

State Department spokesman Robert Wood said a launch would be a "clear violation" of a UN Security Council resolution approved after Pyongyang's first nuclear test, in 2006.

Two US defence officials told AFP in Washington that Pyongyang appeared to have moved a long-range missile to its new launch site at Dongchang-ri along its northwestern coast.

But any launch would likely be weeks away given North Korea's technical capacity, said one of the officials, who asked to remain anonymous.

In April the North fired a Taepodong-2 rocket over Japan from another site on the east coast. It said the launch was to put a satellite in orbit but other nations saw it as a disguised missile test.

Diplomats at the United Nations Security Council are discussing a new resolution which could impose fresh sanctions on the North.

US envoy Susan Rice said there had been progress in talks with her counterparts from Britain, China, France, Russia, Japan and South Korea when they met Monday.

"I think we are making progress and I am hopeful that in due course we will be producing a very worthy and strong resolution," she said.