By Stephanie Nebehay

GENEVA (Reuters) - 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 his 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 said. "This is useless, just a gesture for the presidential election."

"There is no meaning, no sincerity," So added.

As a candidate, U.S. President Barack Obama made unfulfilled campaign promises to meet the leader of the Democratic People's Republic of Korea (DPRK), he said.

North Korea conducted a fourth nuclear test in January and launched a long-range rocket in February, triggering tougher international sanctions and the adoption of a more hardline position by South Korean President Park Geun-hye.

So, who is also North Korea's ambassador to the U.N.-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 will never use them first," So said. "If the United States use their nuclear weapons first, then we have to use also that one."

"If the United States gives up their hostile policies and changes their attitude, then we also (can) have relations as a normal country," So said. "To South Korea, we proposed high-level military talks but South Korea refused now."

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South Korea dismissed on Monday a North Korean proposal for military talks as "a bogus peace offensive" and said it was formally rejecting the overture because it lacked a plan to end the North's nuclear program.

So said that North Korea would not share nuclear technology with other countries. "As a responsible nuclear state, we keep and observe the obligations of non-proliferation of nuclear technology".

(Reporting by Stephanie Nebehay; Editing by Tom Heneghan)