South Korea said Wednesday it would accelerate the development of longer-range ballistic missiles that could cover the whole of North Korea in response to a third nuclear test by Pyongyang.

"We will speed up the development of ballistic missiles with a range of 800 kilometres (500 miles)," Defence Ministry spokesman Kim Min-Seok told reporters.

In October last year, South Korea reached a deal with the United States to almost triple the range of its missile systems -- with Seoul arguing it needed an upgrade to counter the North's missile and nuclear programmes.

The United States has 28,500 troops in South Korea and guarantees a nuclear "umbrella" in case of any atomic attack. In return, Seoul accepts limits on its missile capabilities.

Prior to the October agreement, the South was restricted to missiles with a range of 300 kilometres.

The extension will not only bring the whole of North Korea within reach of Seoul's rockets, but also parts of China and Japan.

Some experts have suggested it would provide the South with a pre-emptive strike facility against the North's nuclear installations.

Kim said the South would also speed up the deployment of a "kill chain" system capable of detecting, targeting and destroying North Korean missiles.

"The military is closely watching the North in case of further provocative acts," he said.

Following North Korea's nuclear test on Tuesday, the head of South Korea's intelligence agency warned that Pyongyang may well carry out a further test or a ballistic missile launch in the coming days or weeks.