The South Korean government has warned that North Korea is preparing for another nuclear test.
The UN Security Council plans further sanctions against Pyongyang after it carried out its fifth nuclear test last Friday.
"Assessment by South Korean and US intelligence is that the North is always ready for an additional nuclear test in the Punggye-ri area," South Korean defence ministry spokesman Moon Sang-gyun told a news briefing.
Punggye-ri, near the northeastern coast, is the site of the North's five nuclear explosions.
"North Korea has a tunnel where it can conduct an additional nuclear test," Mr Moon said.
South Korea's Yonhap News Agency reported earlier that North Korea had completed preparations for another nuclear test, citing South Korean government sources who said the North may use a previously unused tunnel at its mountainous test site.
It did not elaborate on what activities had been detected at the Punggye-ri nuclear test site.
A US special envoy for the isolated state, Sung Kim, will travel to Seoul today.
Mr Kim met with Japanese officials yesterday and said the US may launch unilateral sanctions against North Korea, echoing comments by US President Barack Obama on Friday in the wake of the test.
A push for further sanctions was "laughable", North Korea said yesterday, vowing to continue to strengthen its nuclear power.
