North Korea appears to have developed a nuclear weapon that can be delivered by missile, according to the Pentagon’s intelligence agency, a milestone in the country’s nuclear programme that could heighten the already tense atmosphere on the Korean peninsula.

The Defence Intelligence Agency said in a report prepared last month that it had “moderate confidence” the North Koreans now had a nuclear device small enough to be “capable of delivery by ballistic missiles”, the first time a US intelligence agency has come to that conclusion.

However, the report still insisted that “the reliability will be low” for such a missile, an indication of the considerable technical barriers that North Korea still faces, including developing dependable medium-range and long-distance missiles.