A senior North Korean official has told a visiting Japanese academic that the country may soon test a nuclear device, according to media reports.
N Korea's nuclear capabilities have long been a point of contention
Pak Hyon-jae, deputy head of the Institute for Disarmament and Peace, told Japanese academic Yasuhiko Yoshida that a test was "unavoidable".
But South Korea cast doubt on the claim, saying it had detected no signs of preparations for a test.
North Korea accused the US of "making a fuss" by talking about a possible test.
The long-running row over North Korea's nuclear capabilities has escalated in recent weeks, amid speculation that Pyongyang might be preparing to test a nuclear device.
North Korea said it had nuclear weapons in February, but the claim has not been verified.
The head of the International Atomic Energy Agency, Mohammed ElBaradei, said on Sunday that Pyongyang may already have six nuclear weapons.
Yasuhiko Yoshida, a North Korean specialist at Osaka University in Japan, said he was told by Pak Hyon-jae that "a plutonium-based test is unavoidable" - an "indispensable" measure to prove North Korea's nuclear capabilities.
The world would "soon know about a nuclear test," he quoted Mr Pak as saying.
Mr Yoshida was speaking to various media sources on Tuesday, about comments Mr Pak made to him during an unarranged telephone call earlier this month.
But also on Tuesday, a South Korean official told Yonhap news agency that Seoul had detected no signs that North Korea was preparing a nuclear test, nor had it received any US intelligence to that effect.
In North Korea itself, the state-run Rodong Sinmun newspaper said that Washington had branded North Korea a "nuclear criminal" so as to "stifle" the country.
"The US is making a fuss saying it was notifying the International Atomic Energy Agency, Japan and other related countries of its own opinion that our republic may conduct an underground nuclear test in June," the newspaper said, according to the official KCNA news agency.
It neither confirmed nor denied that the North was planning such a test.
The international community is currently trying to persuade Pyongyang to return to six-party negotiations over its nuclear ambitions.
The talks have been stalled since last June, with North Korea and the US involved in an escalating war of words.
North Korea said it wanted to clarify the United States position before making a final decision on returning to the talks.