North Korea said yesterday it would attack South Korean loudspeakers and other propaganda facilities along its border, warning it could even turn Seoul into a "sea of flame".

The general staff of the Korean People's Army made the threat in a declaration carried by the official Korean Central News Agency.

It said the North would launch an all-out military strike to blow up any propaganda facilities along its border and that its retaliation would be "a merciless strike foreseeing even the turn of Seoul... into a sea of flame".

Tensions have risen since Seoul blamed the North last month for the sinking of a navy ship that killed 46 sailors. North Korea denies involvement.

In 2004 the rival Koreas ended decades of propaganda campaigns as relations warmed following a landmark summit in 2000. But South Korea resumed radio broadcasts to the North last month and installed a dozen propaganda loudspeakers along the border. The resumption of psychological warfare was part of punitive steps taken against the North over the warship sinking.

South Korean defence minister Kim Tae-young announced the launch of the loudspeaker broadcasts at a parliamentary hearing on Friday.

South Korea has officially asked the UN security council to punish North Korea for what Seoul said was a North Korean torpedo attack on the 1,200-ton Cheonan warship.

A multinational investigation led by South Korea concluded last month that North Korea was responsible.

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 following the Korean War.

Seoul, South Korea's capital of more than 10 million people, is considered vulnerable because it is well within the range of North Korean artillery.