North Korea today vowed to strengthen its nuclear weaponry in the face of what it branded US hostility, as world leaders intensified pressure on Pyongyang over the sinking of a South Korean warship. A foreign ministry spokesman said recent developments underscore the need for the North to bolster its nuclear deterrent in a newly developed way to cope with persistent US hostility and its military threat. However, no details were given. Meanwhile, a South Korean official said the North's statement indicates that it would start developing nuclear weapons based on highly enriched uranium. <br/>