South Korean officials have said there are no indications that the North is about to carry out a nuclear test.
A defence ministry spokesman said while activity had been detected at the Punggye-ri underground test site, it appeared to be routine.
South Korean Unification Minister Ryoo Kihl-jae had earlier said there were indications a test was being prepared.
Meanwhile the North has said it is withdrawing all its workers from the shared Kaesong industrial complex.
UN Secretary General Ban Ki-moon said such a nuclear test would be a "provocative measure", and warned that North Korean cannot continue "confronting and challenging the authority of the Security Council and directly challenging the whole international community".
With tension rising on the Korean peninsula, he urged Pyongyang to abide by UN resolutions against its nuclear programme, saying this was "an urgent and honest appeal from the international community, including myself".
Russian President Vladimir Putin said that as a neighbour of North Korea his country was worried by the rising tensions, urging a return to negotiations to "start to resolve the problems that have piled up for many years".
China has also called for calm and dialogue.
'No unusual movement'
On Monday, Mr Ryoo was asked by South Korean MPs about a news report which quoted officials as saying an increased movement of vehicles and personnel had been seen at North Korea's Punggye-ri site.
He replied that there were "such signs", but that he could not elaborate on specific intelligence.
But the defence ministry spokesman Kim Min-seok later said that while such activity had been observed it was nothing unusual, and there was "no indication that a nuclear test is imminent".
But he added that the North was always in a state of readiness to do so.
The Associated Press quoted Mr Ryoo as saying he had misspoken, but the confusion is being seen as an indication of just how tense the situation has become on the Korean peninsula over recent weeks.
Pyongyang's announcement that it was withdrawing its more than 50,000 workers from the joint Korean Kaesong complex came as a senior official Kim Yang-gon visited the site.
The complex, just over the border in the North, is staffed by Northerners but funded and managed by South Korean firms, and had been one of the last remaining symbols of inter-Korean co-operation.
Pyongyang has already banned South Koreans from entering, but Kim Yang-Gon, secretary of the party's Central Committee, said Pyongyang would now "temporarily suspend the operations in the zone and examine the issue of whether it will allow its existence or close it".
"How the situation will develop in the days ahead will entirely depend on the attitude of the South Korean authorities," the North's KCNA news agency quoted him as saying, blaming "military warmongers" in the South for the closure.
The United Nations imposed tough sanctions on North Korea last month following its third nuclear test. Pyongyang has responded by issuing almost daily threats to use nuclear weapons and saying it would restart its nuclear reactor.
The North has also shut down an emergency military hotline between Seoul and Pyongyang. Last week it warned it would not be able to guarantee the safety of foreign embassy staff after 10 April, and that countries should begin evacuating their diplomatic staff.
The BBC's John Sudworth in Seoul says North Korea's state media have been broadcasting a continuing diet of war and retribution with programmes about biochemical war, nuclear war and military preparations dominating the listing.
But some analysts have suggested that the rhetoric is in large part designed to shore up the standing of a young, inexperienced leader, Kim Jong-un, he adds.