Seoul, South Korea ? In an unusually explicit threat, North Korea warned Tuesday it would take “an immediate physical retaliatory step” against the United States if Washington imposed a blockade in the ongoing standoff over Pyongyang’s nuclear program.

Secretary of State Colin Powell appeared to shrug off the warning and expressed confidence that U.S. efforts to get North Korea to give up its nuclear ambitions are working.

“I am pleased at the unity we have been able to achieve with the countries in the region. Everybody is saying the same things to the North Koreans with respect to the unacceptability of their actions, that they want a Korean peninsula without nuclear weapons,” Powell told reporters traveling with him to Cambodia for a regional security meeting.

“We will not be intimidated by their rhetoric or their actions,” Powell added.

The United States is exploring various means short of sanctions or a blockade to pressure North Korea. One topic discussed last week with allies at a meeting in Madrid was tightening international laws on the sale of weapons of mass destruction and their delivery systems.

Powell said Tuesday that the goal was to explore ways of making it more difficult to transfer weapons or their components “in the airways and seaways and landways of the world.”

In a related but separate effort, the United States also is pressing for a crackdown on North Korea’s narcotics and counterfeit currency trafficking, which are believed to generate cash for Kim Jong Il’s impoverished regime.

North Korea’s official Rodong Sinmun newspaper said the U.S. efforts were part of a war plan. In a commentary carried by the nation’s official news agency, North Korea said a retaliatory step would be carried out “against the U.S. once (the North) judges that its sovereignty is infringed upon by Washington’s blockade operation.” North Korea also threatened Japan.