South Korea and Thailand criticized North Korea on Sunday, saying the country's nuclear test threatens world peace and stability and harms efforts to prevent atomic proliferation.

The two nations' leaders discussed Pyongyang's latest nuclear blast on the sidelines of a summit between South Korea and Southeast Asian countries being held amid heavy security.

The event was planned months ago, but North Korea's underground nuclear test and a series of short-range missile launches last week threatens to steal the limelight from economic matters, the main focus of the agenda.

South Korean President Lee Myung-bak and Thai Prime Minister Abhisit Vejjajiva agreed that the test goes against international efforts to prevent nuclear proliferation and "undermines peace and stability not only in East Asia but also in the whole world," Lee Dong-kwan, the South Korean president's chief spokesman, told reporters.

They also agreed to exert diplomatic pressure to assure North Korea complies with U.N. Security Council resolutions and "promptly returns to six-party talks" aimed at ridding it of nuclear weapons.

The summit venue of Seogwipo — on the island of Jeju off the southern coast — is the South Korean city farthest away from the North. Still, the nervous South Korean government is taking no chances, positioning a surface-to-air missile outside the venue aimed toward the north.

Some 5,000 police officers, including approximately 200 commandos, and special vehicles that can analyze sarin gas and other chemicals have been deployed nearby, security authorities said in a press release. Marines, special forces and air patrols also kept watch on the island.

Leaders of the 10 members of the Association of Southeast Asian Nations began arriving for the two-day summit, which officially begins Monday and commemorates 20 years of relations between South Korea and the bloc. South Korea's president planned to use Sunday for individual meetings with ASEAN leaders.

But concerns about North Korea's most recent bout of saber-rattling loomed. South Korean officials said Saturday that spy satellites had spotted signs that the North may be preparing to transport a long-range missile to a launch site.

The North has attacked South Korean targets before, bombing a Korea Air jet in 1987 and trying to kill then-President Chun Doo-hwan in Myanmar in 1983. But Pyongyang has largely abandoned such overt tactics in the past two decades.

The U.N. Security Council is still weighing how to react to the North's belligerent moves that have earned Pyongyang criticism from the U.S., Europe, Russia and even the North's closest ally, China.

U.S. Defense Secretary Robert Gates said Saturday that North Korea's progress on nuclear weapons and long-range missiles is "a harbinger of a dark future" and has created an urgent need for more pressure on the reclusive communist government to change its ways.

Gates, speaking at an annual meeting of defense and security officials in Singapore, said Pyongyang's efforts pose the potential for an arms race in Asia that could spread beyond the region.

An incensed North Korea said last month that it was quitting the six-party negotiations after the U.N. Security Council condemned its April 5 rocket launch, widely believed to be a test of its long-range missile technology. The Security Council has imposed sanctions against the North over its first nuclear test in October 2006.

The six-party framework, which began in 2003, consists of the two Koreas, China, Japan, Russia and the United States.

ASEAN consists of Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Vietnam.

In addition to the summit, a gathering of South Korean and Southeast Asian business leaders began Sunday with addresses by Lee and Abhisit, who both called for further cooperation to overcome the global economic crisis.