South Korean Foreign Minister Ban Ki-moon has warned North Korea that a nuclear test would pose a serious international threat.
South Korea has increased its monitoring of the North
But Mr Ban said that while officials were keeping a close watch on the North's activities, a nuclear test was viewed only as a possibility.
Last week US media said officials had observed suspicious activity at a suspected nuclear test sight.
Tensions remain high in the wake of Pyongyang's missile tests in July.
North Korea provoked international concern when it launched seven missiles, including a new long-range weapon capable of hitting parts of the US.
Multilateral talks aimed at persuading the North to give up its nuclear ambitions are currently deadlocked, with no sign of willingness from the North to engage in dialogue.
'Far more serious'
Mr Ban told reporters a North Korean nuclear test would have serious consequences.
"If North Korea conducts a nuclear test, it would be a far more serious situation than the missile tests, something that would shake the foundation of international non-proliferation efforts and pose a very serious threat," he said.
But more information was needed, he added.
"So far, it is seen as a matter of possibility, and related countries are sharing intelligence and keeping a close eye on the North's activities."
Last week's report by ABC news quoted an unnamed State Department official as saying a nuclear test was a "real possibility", based on observations by intelligence officials of vehicle movement at the suspected underground test site.
Some officials have played down the report and South Korea said last week there was no clear evidence of an impending test.
But Seoul has increased personnel at a seismic monitoring centre, which could detect tremors that indicate a nuclear explosion.
North Korea has claimed to have nuclear weapons, but is not yet known to have tested one.