Fears are growing that North Korean leader Kim Jong Un plans to push ahead with a threat to explode a nuclear device in defiance of the United Nations.

He convened top security and foreign affairs officials and ordered them to take “substantial and high-profile important state measures,” state media have said.

The meeting of top officials led by Kim makes clear that he backs Pyongyang’s defiant stance in protest at a UN Security Council punishment for a rocket launch that the North carried out last month.

Last week, the council condemned North Korea’s launch of a long-range rocket as a violation of a ban against nuclear and missile activity. The council, including North Korea ally China, punished Pyongyang with more sanctions and ordered the regime to refrain from a nuclear test – or face “significant action”.

North Korea responded by rejecting the resolution and maintaining its right to launch a satellite into orbit as part of a peaceful civilian space programme. It also warned that it would keep developing rockets and testing nuclear devices to counter what it sees as US hostility.

North Korea is estimated to have enough weaponised plutonium for four to eight bombs. However, it is not known whether North Korean scientists have found a way to build nuclear warheads small enough to mount on a long-range missile.

Experts say regular tests are needed to perfect the technique, and another atomic test could take the country closer to its goal of building a warhead to be mounted on a missile to strike the US.

Satellite photos taken this week show that over the past month, roads have been kept clear of snow and that North Koreans may have been sealing the tunnel into a mountainside where a nuclear device would be detonated.

The US, South Korea and other countries have warned North Korea not to go ahead with a nuclear test, saying that would only deepen the country’s international isolation.