US Ambassador John Bolton, meeting with the UN Security Council yesterday, said the North has a history of intimidating other nations.
"They're not going to be successful with us," he said on CBS television's The Early Show.
Bolton refused to rule out military action, including a naval blockade, but emphasized that US President George W. Bush wants to resolve the matter using peaceful means.
The North has insisted that Washington hold direct talks with Pyongyang to resolve the standoff over its nuclear program. But the US ambassador in Seoul, Alexander Vershbow, said Monday's report that the North conducted the underground nuclear test would make the possibility of such talks more difficult.
Vershbow insisted that six-party talks -- also involving China, Japan, Russia and South Korea -- were still the best approach.
"The North Korean nuclear issue is not a bilateral problem but rather a problem between North Korea and all of its neighbors as well as the US," he told reporters.
The UN Security Council was discussing a US draft resolution that aims to curb the North's nuclear and ballistic missile programs, prohibit all trade in military and luxury goods, and crack down on illegal financial dealings.
However, the potency of possible new sanctions is a wild card in the efforts of the UN Security Council, the US and its allies to contain North Korea's nuclear ambitions and coax it back to negotiations.
The US, Japan and others have slapped a series of sanctions on Pyongyang over its nuclear program in recent years.
And the countries had indicated that similar consequences would be in store over a nuclear test, long before Monday's still-unconfirmed detonation.
Actions already taken range from blacklisting North Korean banks and restricting port entry of its ships to backing a global ban on trading some military technology with the North.
In the wake of the crackdown, trade with Japan alone tumbled 85 percent, to a paltry US$195 million last year, from 2001.
Yet North Korea went ahead and tested anyway.
But increasing sanctions may be all that's left. Military action or a strategic airstrike against North Korea is riskier than ever in the face of threatened retaliation with atomic weapons.
Tougher measures hinge on whether China and Russia -- the North's closest allies -- support them.
Measures under consideration at the UN include international inspection of all cargo to and from North Korea to limit the proliferation of nuclear, chemical and biological weapons, and blanket bans on luxury and military goods and any material that could be used in the production of weapons of mass destruction.
A US draft resolution contains tough new proposals from Japan to ban all countries from allowing in North Korean ships or aircraft carrying arms, nuclear or ballistic missile-related material or luxury goods.