Ban Ki-moon urged UN Security Council to take "appropriate action" against North Korea.
UN Secretary-General Ban Ki-moon on Friday urged the Security Council to take "appropriate action" in response to North Korea's fifth nuclear test.
Ban condemned the underground nuclear test "in the strongest possible terms," branding it "yet another brazen breach of the resolutions of the Security Council."
"I count on the Security Council to remain united and take appropriate action. We must urgently break this accelerating spiral of escalation," he told reporters.
Ban spoke ahead of an emergency meeting of the council requested by the United States and Japan to discuss Pyongyang's fifth and most powerful nuclear test to date.
North Korea's state media said the test, which came after a series of ballistic missile launches, had realized the country's goal of being able to fit a miniaturized warhead on a rocket.
The blast at the Punggye-ri nuclear site had a yield of 10 kilotons, according to experts.
The council has strongly condemned North Korea's missile launches and vowed earlier this week to take "further significant measures" against Pyongyang.
Japan is calling for new sanctions, but China, Pyongyang's ally, has repeatedly stressed the need to avoid an escalation of tensions on the Korean peninsula.
British Ambassador Matthew Rycroft said the council could decide to push for full implementation of existing sanctions against North Korea or add names of companies and individuals to a sanctions blacklist.
"There could be a tightening up and a strengthening of the sanctions regime. All those things are possible," he said.
North Korea has been hit by five sets of UN sanctions since it first tested a nuclear device in 2006.
The council in March adopted the toughest sanctions resolution to date targeting North Korea's trade in minerals and tightening banking restrictions, after Pyongyang carried out its fourth nuclear test.