The South Korean military is closely watching its neighbour for possible signals of an imminent long-range missile launch which has been reported in the media.

Seoul said there are no indications as yet but admitted Pyongyang may decide to press on with the missile test "abruptly". During a routine press briefing, South Korea's defence ministry spokesperson Kim Min-Seok said: "North Korea is likely to do it abruptly when they launch important provocative acts in the future. Our military is keeping close tabs on the signs of North Korea's long-range missile launch."

The spokesperson added: "A no-sail zone is required internationally because North Korea's past long-range missiles or rockets flew to the east coast of the Philippines. But I have not heard so far on that."

In the past, North Korea usually has declared a no-fly and no-sail zone whenever there is a missile launch. The notification is mandatory according to UN regulations but the defiant North has a track record of violating norms.

Earlier, Japan's Kyodo news agency quoted an unnamed Tokyo official as saying that Pyongyang may be preparing for the missile launch as early as at the start of February 2016. The Japanese government official cited satellite images of North's Tongchang-ri missile site. The South Korean defence ministry spokesperson neither confirmed nor denied the report.

North Korea had earlier claimed to have conducted a sophisticated hydrogen bomb test on 6 January forcing the UN Security Council members to discuss a fresh round of sanctions. The impoverished North is already under several economic clampdown due to its ambitious nuclear activities.