South Korea plans to complete its own missile defence system against North Korea in three years, officials said Sunday amid rising tensions over Pyongyang's new test-launch preparations.

Officials at Seoul's defence ministry said South Korea was pushing to establish its own air defence unit, exclusively to detect and intercept North Korean ballistic missiles, by 2012.

The unit, to cost a total of 300 billion won (214 million dollars), will complete an ambitious air and missile defence project which has been pushed for since 2006, they said.

Seoul plans to buy new radars which can detect objects up to 1,000 kilometres (600 miles) away for the new system, which will put the North's missiles under close watch around the clock, they said.

The two Koreas are still technically at war since the 1950-1953 Korean conflict ended in a fragile armistice.

North Korea has short-range Scuds and Rodongs with a range of 1,300 kilometres, while actively developing longer-range Taepodong missiles that could reach the United States.

Scuds and Rodongs put all of South Korea within range.

In recent weeks, Pyongyang has apparently started assembling its longest-range Taepodong-2 missile and it could be ready for launch late this month, according to media reports in Seoul and Washington.

The Taepodong-2 could theoretically reach Alaska but blew up after 40 seconds when it was first test-fired in July 2006.

South Korea has warned that any launch would bring the North increased isolation and added sanctions. The United States said it would be provocative.

The North has responded furiously to South Korean President Lee Myung-Bak, who took office in February last year and who has linked major economic aid to progress in the communist country's nuclear disarmament.

Late last month, the North said it had scrapped all peace accords with the South, including a 1991 agreement that recognised the Yellow Sea border as an interim frontier off the western coast.

The announcement sparked fears of another inter-Korean clash, as the sea border was the scene of bloody naval battles in 1999 and 2002.

Local media have also reported the number of North Korean guns, mostly 100-mm artillery pieces, sited on western islands and along the coast in the area increased by 30 percent last year from 2007.

The Munhwa Daily quoted an unidentified official of the Joint Chiefs of Staff in Seoul as saying Saturday that the South would bolster its defence on islands near disputed waters in the Yellow Sea.

South Korea in 2007 launched its first Aegis destroyer, which was finally deployed for operational use in December last year.

The King Sejong, the one-billion-dollar 7,600-tonne KDX-III destroyer, adopts the US-built Aegis system that allows a ship to combat multiple surface, underwater and aerial threats.

South Korea plans to deploy a second Aegis destroyer and a third for operational use in 2010 and 2012, according to its navy.

Last year, South Korea began taking delivery of US-made Patriot missiles to replace its ageing Nike ground-to-air missiles and better cope with North Korean missile threats.

Seoul had announced a plan to purchase 48 Patriots by this year, setting 2010 as a target for them to be operational.

The United States, which bases 28,500 troops in South Korea, has upgraded its Patriot batteries here with advanced missiles.