BEIJING (Reuters) - Chinese Foreign Minister Wang Yi told his Japanese counterpart that China opposes "unhelpful" unilateral sanctions on North Korea but will work within the United Nations to formulate a necessary response to its fifth nuclear test.

China has expressed anger with North Korea for its largest nuclear test to date last week, but has not said directly whether it will support tougher sanctions.

China has said it believes sanctions are not the ultimate answer and called for a return to talks.

Wang told Japanese Foreign Minister Fumio Kishida that China opposed the nuclear test, and that it is willing to work with other permanent members of the UN Security Council to "come up with a necessary response to the new changes to the situation on the peninsula", China's Foreign Ministry said late on Wednesday.

"With all sides focused on the authoritative channel of the Security Council, China opposes unilateral sanctions that are unhelpful to resolving the issue," the statement paraphrased Wang as saying, without elaborating.

A U.S. special envoy for North Korea, Sung Kim, met Japanese officials on Sunday and said the United States may launch unilateral sanctions against North Korea, echoing comments by U.S. President Barack Obama last Friday.

Japan's Foreign Ministry said Kishida said the nuclear test was unforgivable and a direct, grave threat to Japanese security, and asked China for a constructive response as a responsible permanent member of the Security Council.

China is isolated North Korea's most important diplomatic backer and biggest trading partner, but frustrated with repeated nuclear and missile tests it has signed on to increasingly tough UN sanctions and insisted it is complying with them.

(Reporting by Ben Blanchard; Additional reporting by Linda Sieg in Tokyo; Editing by Nick Macfie)