"We're working up at the UN with our Security Council colleagues on what sort of shape these consequences should take," State Department spokesman Ian Kelly told reporters.

"One we're looking at a broad range of possible responses to it, including possible sanctions," he said, the first time a US official has spoken on the record of possible sanctions.

He said the Russians, one of the five Security Council permanent members, "understand the need for a strong message to send to North Korea," that "their actions are unacceptable and there must be consequences for these actions."

But he conceded Washington is still seeking clarification from Moscow about its stand.

In Tokyo, Japan's foreign ministry said Russia is ready to discuss imposing sanctions on North Korea over the nuclear test, despite reports suggesting it was opposed to such measures.

The ministry said in a press statement that Russian Foreign Minister Sergei Lavrov had expressed his country's willingness to impose sanctions, in a telephone conversation with his Japanese counterpart, Hirofumi Nakasone.

The ministry said the interview was held at Nakasone's request.

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