SEOUL (AFP) - North Korea vowed Wednesday to bolster its nuclear deterrent, describing an exercise by United States and South Korean troops as preparation for a "war of aggression" against the communist state. The 10-day drill, which began on Tuesday, showed the US was not committed to dialogue over nuclear disarmament, a foreign ministry spokesman said in a statement published by the North's official Korean News Agency. "The prevailing situation goes to prove that the US is not set to realise the denuclearisation of the Korean peninsula through dialogue and negotiations... " he said.