SEOUL, Sept 29 (Reuters) - The U.N. nuclear watchdog's chief said on Friday North Korea's sixth nuclear test conducted on Sept. 3 showed the isolated country has made "rapid progress" on weapons development.

International Atomic Energy Agency Director Yukiya Amano said the North's sixth and most powerful test posed a "new threat".

"What is most important for now is for the international community to unite," Amano reporters after a meeting with South Korean Foreign Minister Kang Kyung-wha during a visit to Seoul. (Reporting by Christine Kim; Editing by Paul Tait)