BEIJING (BBC) -- A high-level Chinese envoy has returned from North Korea expressing optimism about the mounting nuclear crisis.

Tang Jiaxuan, who met North Korea's leader Kim Jong-il, said his visit had "not been in vain".

China is North Korea's closest ally and has publicly warned the North not to test another weapon.

Tang was speaking to U.S. Secretary of State Condoleezza Rice, who has stepped up calls for UN sanctions against the North to be fully implemented.

Rice, who is in China as part of an Asian tour to rally support against North Korea, held talks with Chinese Foreign Minster Li Zhaoxing on Friday.

Rice said the two discussed implementing UN sanctions imposed after Pyongyang's 9 October nuclear test and described the talks as "very fruitful".

The meeting comes amid concern North Korea is planning a second detonation, and reports that China has threatened to cut off vital oil supplies if it goes ahead.