South Korea has conducted its first live-fire drill for an advanced air-launched cruise missile that would strengthen its pre-emptive strike capability against North Korea in the event of crisis, according to Seoul's Defence Ministry.

Key points: A pre-emptive strike is seen as the most realistic of Seoul's military options

A pre-emptive strike is seen as the most realistic of Seoul's military options The Taurus missile can reportedly avoid detection by radars

The Taurus missile can reportedly avoid detection by radars South Korea says it has detected radioactivity after North's nuke test

The missile, manufactured by Germany's Taurus Systems, has a maximum range of 500 kilometres and is equipped with stealth characteristics that will allow it to avoid radar detection before hitting North Korean targets.

On Wednesday, the South Korean military said the Taurus missile — fired from an F-15 fighter jet — travelled through obstacles at low altitudes before directly hitting a specific target off the country's western coast.

South Korea has been accelerating efforts to ramp up its military capabilities in face of a torrent of nuclear weapons tests by North Korea, which on September 3 conducted its sixth and most powerful nuclear test to date.

Shortly after the nuke test, Seoul announced it had reached an agreement with Washington to remove the warhead weight limits on South Korean ballistic missiles, which under a bilateral guideline could be built for a maximum range of 800 kilometres.

The Taurus missile can reportedly fly up to 500 kilometres without being detected. ( AP: South Korean Defence Ministry )

A pre-emptive strike against Pyongyang's leadership would be difficult to undertake, but it is widely seen as the most realistic of the limited military options Seoul has to deny a nuclear attack from its rival.

The North said its latest nuclear test was a detonation of a thermonuclear weapon built for its developmental intercontinental ballistic missiles that were flight tested twice in July.

On Tuesday, the United Nations unanimously voted in favour of stepping up sanctions against Pyongyang, which North Korea "categorically rejected".

Meanwhile, South Korea said it found a small amount of radioactivity in air samples collected days after the North's test.

The Nuclear Safety and Security Commission said the discovery of the xenon-133 isotope is linked to the recent test but it could not verify exactly what kind of bomb was detonated because several other isotopes that typically accompany a nuclear explosion were not found.

Those isotopes could show if the bomb tested on September 3 was a plutonium or uranium device, according to the South Korean agency.

It said it also has not found traces of tritium, which accompany a test of a thermonuclear, or hydrogen, bomb.

North Korea did a poor job obscuring its first nuclear test in 2006, when xenon and krypton isotopes detected in the atmosphere allowed scientists to conclude that the country had used a plutonium-fuelled device.

The country has since improved the design of its nuclear tests to make radioactivity less detectable from a distance.

North Korea has been launching ballistic missiles at a record pace. ( AP/KCNA )

AP