The head of the UN watchdog on nuclear testing says North Korea is not successfully showing the world that it's sincere about denuclearization. 

Lassina Zerbo, the Secretary-General of the UN Comprehensive Nuclear-Test-Ban Treaty Organization(CTBTO), made the remark on Monday during an interview with the Wall Street Journal(WSJ) in Seoul. 

Pointing to the North’s display of a nuclear test site destruction before invited foreign journalists in May, Zerbo said blowing up an entrance isn’t enough to say all the tunnels are destroyed, adding another can be opened elsewhere. 

He urged the North to verify to build trust, noting it's not too late for the North to let in international inspectors. He also called on the regime to sign the nuclear test-ban treaty. 

During a meeting with South Korean Foreign Minister Kang Kyung-wha in Seoul on Monday, Zerbo said he supports South Korea’s efforts to bring permanent peace to the Korean Peninsula, vowing his organization’s full support for it.