North Korea is "increasingly belligerent", the US top military officer, Admiral Mike Mullen, said Monday, adding there was no reason to disbelieve the Asian nation's claim it carried out a nuclear test.
"They grow increasingly belligerent," Mullen told CNN. "While it will take us a couple of days to verify this test, certainly there's no indication that it wasn't as they say.
"We weren't surprised because of recent statements by North Korean leadership they intend to do this," said Mullen, who is chairman of the Joint Chiefs of Staff.
North Korea on Monday said it tested a nuclear bomb many times more powerful than its first in 2006, prompting UN Security Council members to call an emergency session.
Mullen emphasized the role of diplomacy in resolving tensions on the Korean peninsula, but said recent developments were part of a pattern of worrying behavior.
"As you know, they also recently... unsuccessfully launched potentially an intercontinental ballistic missile," Mullen said.
"I believe they increasingly isolate themselves from the international community," he said, adding that reactions from Russia, China, South Korea and Japan would be vital.
"The countries involved in that are absolutely critical as we move forward to address this increasingly belligerent challenge from North Korea.
"Obviously we've got forces deployed to that part of the world, we have had for a significant period of time, we've got over 25,000 troops who are stationed in South Korea."
His comments follow a statement from US President Barack Obama who on Monday called North Korea's nuclear and missile tests "a matter of grave concern" and urged international action against the isolated state.
"These actions, while not a surprise given its statements and actions to date, are a matter of grave concern to all nations," Obama said in a statement.
Obama said that "the danger posed by North Korea's threatening activities warrants action by the international community."
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