SEOUL, South Korea (AP) — North Korea vowed Saturday to launch an all-out attack against South Korean loudspeakers and other propaganda facilities along their heavily fortified border, warning it could even turn Seoul into a "sea of flame."

The rival Koreas ended decades of propaganda campaigns in 2004 as their relations warmed. However, South Korea resumed radio broadcasts to North Korea last month and installed a dozen propaganda loudspeakers along the border to punish the North for allegedly sinking a South Korean warship.

South Korean Defense Minister Kim Tae-young told a parliamentary hearing Friday that loudspeaker broadcasts would begin after the U.N. Security Council decides on any new measures against the North, Yonhap news agency reported.

South Korea has asked the U.N. Security Council to punish the North for what Seoul says was a North Korean torpedo attack on the 1,200-ton Cheonan warship that killed 46 sailors.

A multinational investigation led by South Korea concluded last month that North Korea was responsible. The North has denied responsibility and threatened to respond to South Korean retaliatory measures with war.

The General Staff of the Korean People's Army said in a statement Saturday that North Korea would launch an "all-out military strike" to blow up any propaganda facilities along the border, and that its retaliation would be "a merciless strike foreseeing even the turn of Seoul ... into a sea of flame."

The statement was carried by the official Korean Central News Agency.

Seoul, South Korea's capital of over 10 million people, is just 37 miles (60 kilometers) south of the border, well within North Korean artillery range.

The North's military earlier warned it would fire at any propaganda facilities installed in the Demilitarized Zone that has separated the two Koreas since the end of the 1950-53 Korean War, which concluded in a truce, not a peace treaty.

South Korea's Defense Ministry said it had no immediate comment on the threat from North Korea.

South Korean troops increased their vigilance, although no unusual North Korean military movements were detected, an officer at Seoul's Joint Chiefs of Staff said. He asked not to be identified because he was not authorized to speak to the media.

North Korea has for years threatened the South with destruction, though it has never followed through with an all-out military assault since the 1953 armistice was concluded.

In 1994, the North threatened to turn Seoul into a "sea of fire" after talks with South Korea collapsed. In 2008, it also warned the South that "everything will be in ashes, not just a sea of fire."

North Korea keeps two-thirds of its 1.2 million-strong military near the border. The U.S. has about 28,500 troops in South Korea to deter possible North Korean aggression, while South Korea has a 655,000-member military.

On Friday, the North's National Defense Commission, headed by North Korean leader Kim Jong Il, accused Seoul of "infringing upon the dignity and security" of North Korea and criticized it for not allowing North Korean inspectors to visit the South to probe the ship sinking.

In Washington, State Department spokesman P.J. Crowley appeared skeptical of the North's position. "If North Korea wants to investigate the sinking of the Cheonan, as it indicated it might, it might start by taking an inventory of its torpedoes," Crowley said Friday, according to a State Department transcript.