GENEVA (REUTERS) - North Korea is ready to resume six-party talks on its nuclear programme but must maintain its readiness in the face of joint US-South Korean military exercises, a senior envoy in Geneva said on Thursday.

So Se Pyong, the Democratic People's Republic of Korea's (DPRK) ambassador to the United Nations in Geneva, told Reuters that his country was not planning a nuclear or missile test.

In a wide-ranging interview, he said that reports about the ill-health of its leader Kim Jong Un were "fabricated rumours" and that it was not clear whether the United States was willing to negotiate the release of three detained Americans.

Russian Foreign Minister Sergei Lavrov said after talks with North Korea's foreign minister in Moscow on Wednesday that he saw a possibility that stalled talks on Pyongyang's nuclear programme could resume, but it would take time.

"For the six-party talks we are ready, and as far as I think, China and Russia and the DPRK are ready," Mr So said in the rare interview in the DPRK's mission overlooking Lake Geneva.

"But America, they don't like that kind of talks right now. Because America does not like that, so that's why the countries like South Korea, Japan also are not ready for those talks."

North Korea promised to abandon its nuclear programme in 2005 but appeared to renege on the agreement when it tested nuclear devices in 2006 and 2009.

Already heavily sanctioned by the world body for its missile and nuclear tests, it has completed a major overhaul of its rocket launch site, a US think-tank said on Thursday, enabling it to fire larger, longer-range rockets.

Mr So, without being specific, linked North Korea's military preparations to "very serious" US-South Korean exercises earlier this year that he said had deployed nuclear-powered aircraft carriers, submarines and B-52 bombers.

"We have to be alert also, we have to be prepared to make counter measures against that military exercise which are against us."

Asked specifically whether North Korea was preparing a nuclear test or to fire a missile, he replied: "No, no. In case, in case, if they do that kind of joint military exercise, the joint nuclear military exercise against my country continuously, we have to, also."

North Korea's nuclear weapons programme protected it from the United States, Mr So said.

"In case if we give (the weapons) up like other countries, then of course I think they would have attacked us already," he said.

North Korea is not believed to have mastered the technology to miniaturise a nuclear warhead small enough for any of its existing rockets, although analysts say subsequent nuclear tests increase the chance of refining its existing nuclear technology.

It has previously threatened to turn Seoul and Washington into a "sea of flames".

Asked whether North Korea's leader Mr Kim was committed to denuclearisation, Mr So said: "It is the party's policy."

Mr Kim failed to appear at the Supreme People's Assembly last month and state media said he was suffering from "discomfort".

He had been seen walking with a limp since an event in July.

Asked about the nature of his ailment, Mr So said: "That is rumours, fabricated rumours."

He said that media reports Mr Kim may have had surgery on his ankles were wrong.

Three Americans are currently being held in North Korea on charges of crimes against the state. A pro-North Korean daily published in Japan on Thursday quoted one of them appealing to the US government to help to secure their freedom.

"It is true that three Americans are detained in my country now... They came into my country illegally and also they committed some crimes against my country, that is why they are on trial and then they were sentenced," Mr So said.

He added: "I was told that they asked for the government of America to have negotiations on those problems, but I don't know whether America is ready or not to release them or have some understandings or the recognition of those crimes they made."

US special representative for North Korean policy Glyn Davies said on Monday that North Korea has rejected US efforts to discuss the detentions, adding that the secretive state was missing a chance to build relations with Washington.

Mr So disclosed that North Korea has sought closer cooperation on human rights, first with the United Nations for technical assistance, and also through dialogue with the European Union.

"Actually we just gave a hint to have that kind of dialogue to one of the EU member countries, but still they don't give us any feedback, any answer," Mr So said, noting that the EU has taken the lead in criticising North Korea at UN rights forums.

A report by UN investigators this year denounced its system of labour camps holding political prisoners.

US Secretary of State John Kerry, speaking in New York last week, called on North Korea to close its camps.

"Of course every country has prisons. That is true. We have also prisons. But not labour camps they are talking about," Mr So said. "That is a totally fabricated discrimination."

On the politically-charged issue of Japanese nationals abducted by North Korea over the years, Mr So said that he did not know how many abductees there had been, but noted there had been recent contacts on the issue.

"Whether it is a hundred, or 200 or 500, I don't know exactly. We don't know the exact numbers, those are very difficult problems there," Mr So said.

In 2002, North Korea admitted kidnapping 13 Japanese citizens in the 1970s and 1980s to help train spies. Five abductees and their families later returned to Japan.

Officials from North Korea and Japan have met in China on the issue, and Japan's Prime Minister Shinzo Abe, who has made resolution of the dispute a top priority, said this week that North Korea had invited Japanese representatives to Pyongyang.

Mr So said he did not know if a trip would take place.

"I was told also there were some more contact last week," Mr So said. "So anyway, we are ready to solve that problem because we are very frank."