U.S. Republican presidential candidate Donald Trump's proposal to meet North Korean leader Kim Jong-un is a "kind of propaganda or advertisement" in the election race, a senior North Korean official said on Monday.

Trump, in a wide-ranging interview with Reuters in New York last week, said he is willing to talk to the North Korean leader to try to stop Pyongyang's nuclear program, proposing a major shift in U.S. policy toward the isolated nation.

"It is up to the decision of my Supreme Leader whether he decides to meet or not, but I think his [Trump's] idea or talk is nonsense," So Se Pyong, North Korea's ambassador to the United Nations in Geneva, told Reuters on return from Pyongyang after attending the first ruling party congress in 36 years.

"It's for utilization of the presidential election, that's all. A kind of a propaganda or advertisement," he added. "This is useless, just a gesture for the presidential election."

North Korea conducted a fourth nuclear test in January and launched a long-range rocket in February, triggering tougher international sanctions.

So, who is also North Korea's ambassador to the UN-backed Conference on Disarmament, reiterated that his country was prepared to return to stalled six-party talks on its nuclear program.

China and Russia backed the idea, but the United States and its allies South Korea and Japan reject it, he said.

"As a responsible nuclear state ... we never use them first," So said.

"If the United States use their nuclear weapons first, then we have to use also that one." But he added: "As a responsible nuclear state, we keep and observe the obligations of non-proliferation of nuclear technology."