The third stage of North Korea's rocket apparently separated instead of falling together with the second stage about 2,100 km east of Japan into the Pacific as initially thought, sources said Thursday.

Data collected so far by Japan and the United States show the third stage appears to have gone down in waters more than 3,000 km away from the launch site, Musudan-ri, North Hamgyong Province, in North Korea's northeast, the sources said.

The rocket, suspected of being a long-range ballistic missile, reached an altitude of about 500 km after launch Sunday, they said.