Seoul and Pyongyang have cut most cross-border ties after the sinking
North Korea says its military will respond if the UN Security Council rebukes it over an alleged torpedo attack on a Southern warship in March.
In a rare news conference, the North's UN envoy Sin Son-ho demanded that Pyongyang be allowed to send a team to investigate the sinking of the Cheonan.
His comments came a day after North and South gave their versions of events to the UN in separate hearings.
The US said the comments represented more "provocative behaviour".
"We're looking for North Korea to change its unacceptable behaviour, to cease belligerent actions," said White House spokesman Philip Crowley.
"The last thing we want to see is further tension in the Korean peninsula."
End of diplomacy
The South is hoping for tough action from the UN, which has so far expressed "grave concern" but has not blamed either side.
North Korea's UN envoy Sin Son-ho rarely gives news conferences
Mr Sin said the Security Council had so far heard only one side of the story, because the South had turned down the North's requests to be allowed to investigate the incident.
"If the Security Council release any documents against us condemning or questioning us in any document then... [as a] diplomat, I can do nothing," he said.
"The follow-up measures will be carried out by our military forces."
Analysts say the Security Council still has not decided what to do, with China usually reluctant to sanction any move against its allies in Pyongyang.
The warship was sunk in March, killing 46 sailors on board.
Since the sinking, Seoul has suspended trade with its neighbour, to which Pyongyang responded by cutting all bilateral ties.