Tens of thousands rallied in Pyongyang against UN sanctions
The US has confirmed a "probable" nuclear test carried out by North Korea last month.
The explosion yielded several kilotons, more powerful than North Koreas's first nuclear test in 2006, officials said.
It was the United States first official comment on the nature of the May 25 underground test.
The UN Security Council extended sanctions against North Korea last week in response to the test - including the inspection of North Korean ships.
In its confirmation of the test, the US Office of the Director of National Intelligence said "North Korea probably conducted an underground nuclear explosion in the vicinity of Punggye" on 25 May.
Earlier on Monday, tens of thousands of people rallied in the North Korean capital, Pyongyang, against the UN resolution to impose sanctions, state media reported.
Following the resolution the North said it would start enriching uranium and use the plutonium for nuclear weapons.
The US strongly condemned the threat.