North Korean Foreign Minister Ri Su-yong is in New York for a rare trip to participate in a United Nations gathering on climate change. Delivering his keynote address, he said the US is to be blamed for North Korea's controversial nuclear programme.

"In order to remove nuclear threats, we tried dialogue and made efforts through international law, but everything went down the drain. The only thing left was to respond to nukes with nukes," said Ri.

Ri travelled to New York from Beijing amid high tensions in the Korean peninsula ever since Pyongyang conducted its fourth nuclear test followed by a long-range rocket launch.

Ri earlier visited New York in September 2015 to attend the UN General Assembly but this was his first visit since the situation worsened. His trip had led to speculation that he might meet some top US officials but this has been denied.

In his speech, Ri also took the opportunity to dub the US joint military exercises with South Korea as another rehearsal for an attack on Pyongyang. Condemning the UN-led economic sanctions, he added the North would not cave in to the curbs engineered by the US. Ri urged Washington to scale down its "hostile policy".

His remarks have also come at a time when there is a growing concern that the North might press ahead with its fifth nuclear test.

Responding to the reports, US State Department spokesperson John Kirby warned: "We take these threats very seriously. Even when they just say, claim they're going to do something. We take it at face value. It matters to us. Just as critically, we know it matters a heck of a lot to the people in the region and to our allies in South Korea, so we have to take this stuff seriously, and we do."