It warned it would take "merciless self-defensive" measures.
North Korea state media said the Trump administration was "pursuing a policy of military confrontation but this is nothing but a death-bed struggle by those alarmed by the might of the DPRK always emerging victorious".
"Should the United States and its followers try to enforce the naval blockade against our country, we will see it as an act
of war and respond with merciless self-defensive counter-measures as we have warned repeatedly," said KCNA, citing a foreign ministry spokesman.
The US has not indicated any naval blockade - used to enforce sanctions - is imminent.
White House National Security adviser HR McMaster said in September that even 'non-strike' options carried the risk of military escalation.
It comes as Chinese President Xi Jinping, during a visit by South Korea's leader, said on Thursday that war must not be allowed to break out in the region.
"The peninsula issue must, in the end, be resolved via dialogue and consultation," he said.
South Korea's Yonhap news agency said the presidents had agreed to cooperate in applying sanctions and pressure on the North.
UN chief Antonio Guterres also said on Thursday that it was vital Security Council resolutions on North Korea were fully implemented by all countries.
"The worst possible thing that could happen is for us all to sleepwalk into a war," Mr Guterres said during a visit to Tokyo to meet Japan's PM.
North Korea's KIm Jong Un continues to defy the international community with regular missile tests and threats of nuclear strikes against the US if provoked.
Despite President Trump's bombastic language and threats of his own, US secretary of state Rex Tillerson recently offered direct talks with North Korea without pre-conditions.
More follows...