A secretive dissident group committed to overthrowing the North Korean regime has claimed responsibility for an attack on its embassy in Madrid.

The group broke its silence late on Tuesday after being outed by a Spanish judge, who reported that the mastermind behind the February plot was a Mexican citizen who flew straight to the US to offer sensitive information to the FBI.

Adrian Hong Chang is said to be a long-time activist who was spurred to action against the repressive regime after reading the memoir of a North Korean prisoner. Fellow activists said he had been briefly detained in China in the past for his work supporting North Korean defectors.

The 14-page summary issued by Spanish high court judge Jose de la Mata alleges that Chang, along with US citizen Sam Ryu and five South Koreans, were members of a gang of 10 who tied up and interrogated six diplomats before making off with confidential files.

The Cheollima Civil Defence group - whose name refers to a mythical winged horse often used in North Korean propaganda - hit back after the report was published, accusing the US authorities of a deep betrayal that had endangered their lives.