There was more condemnation of North Korea from Russia, EU and the UK, although China - the North's main ally - refused to apportion blame.
A spokesman for the Chinese foreign ministry said that both countries should "do more to contribute to peace".
"What's imperative now is to restart six-party talks as soon as possible," Hong Lei told reporters in Beijing.
Japan's Prime Minister, Naoto Kan, said he had ordered ministers to prepare for any eventuality.
"I ordered them to make preparations so that we can react firmly, should any unexpected event occur," he said.
Russia's foreign minister warned of a "colossal danger", and said those behind the attack carried a huge responsibility.