It warned that President Donald Trump would be taking an “extremely dangerous and big step toward nuclear war.”
The US has not threatened a naval blockade but Secretary of State Rex Tillerson has raised it as a possibility.
North Korea, formally known as the Democratic People’s Republic of Korea and led by tyrant Kim Jongun, vowed to take “merciless self-defensive” measures.
It said US policy of “military confrontation” was nothing but a “deathbed struggle by those alarmed by the might of the DPRK always emerging victorious.”
The threat was issued through the official state news agency.
North Korea last month test-fired a missile which it claimed put the whole of the United States within range.
In September it detonated its sixth and largest nuclear bomb in an underground test.
Kim is in a race to develop a nuclear warhead for his missiles.
Mr Trump has vowed to respond to any North Korean threat with “fire and fury”.
On Tuesday Mr Tillerson offered to begin direct talks with North Korea without pre-conditions.
But he was slapped down by the White House which said no negotiations could be held until North Korea improved its behaviour.
Foreign Secretary Boris Johnson said military options in North Korea “did not look attractive” and favoured tighter economic sanctions.