SEOUL (Reuters) - North Korea conducted its fifth and biggest nuclear test on Friday and said it had mastered the ability to mount a warhead on a ballistic missile, ratcheting up a threat that its rivals and the United Nations have been powerless to contain. Source: [url=http://feeds.reuters.com/~r/Reuters/worldNews/~3/-Khma_aDkB8/us-northkorea-nuclear-idUSKCN11F02N]North Korea's largest nuclear test draws condemnation, threat of more sanctions[/url]