US President George W Bush has accused North Korea of trying to "test the will" of the nations negotiating with it over its nuclear programme.
North Korea said the South could pay dearly for imposing sanctions
"The leader of North Korea likes to threaten," Mr Bush said.
Pyongyang has warned Seoul it will regard its participation in sanctions against the North as a provocation.
The UN Security Council has approved arms and financial sanctions against North Korea following its nuclear test - but is debating how to enforce them.
It has called on all members to state how they plan to implement sanctions by mid-November.
China voted for the resolution but fears harsh sanctions on Pyongyang could squeeze the impoverished regime, risking its collapse - and chaos in its wake.
And Russian President Vladimir Putin has said North Korea should not be driven into a corner if the world wants to rein in its nuclear weapons programme.
He said some negotiators "failed to find the right tone" with the country, adding a solution could be found "with goodwill".
China, Russia, South Korea, the US and Japan have been partnering each other in stalled talks over Pyongygang's nuclear programme.
Mr Bush said on Wednesday that the five countries remained united in their stance on North Korea.
"Our goal is to continue to remind our partners that when we work together, we're more likely to be able to achieve the objective, which is to solve this problem diplomatically," he said.
In its statement on sanctions, North Korea said: "If the South Korean authorities end up joining US-led moves to sanction and stifle, we will regard it as a declaration of confrontation against its own people ... and take corresponding measures."
Mr Bush responded by saying the North Korean leader, Kim Jong-il, "is just testing the will of the five countries that are working together to convince him there is a better way forward for his people".
Meanwhile, South Korean Unification Minister Lee Jong-seok has offered his resignation over his country's handling of North Korea's nuclear test.
Mr Lee was criticised for not having tough enough policies against Pyongyang.
"During this political strife, I thought someone with more talent than I have should come to this position and overcome the problem," he said.
It comes a day after the South Korean Defence Minister, Yoon Kwang-ung, tendered his resignation.
Japan is also debating its handling of the nuclear crisis.
The country's defence chief, Fumio Kyuma, has said the country should not engage in "careless debate" over whether it should possess nuclear weapons.
His comments were in reaction to those made by politicians including Foreign Minister Taro Aso, who suggested Japan should discuss the development of nuclear warheads.
"We have advanced technology and missile capabilities so perhaps we do have the potential to make nuclear arms. But we are not going to do so," he said.