After the United States and South Korea announced new military exercises this year to deter North Korea, the State Department has threatened harsh reactions.
A spokesman in Pyongyang accused the United States of pursuing hostile policies.
Together with their allies, they have brought the situation on the Korean peninsula to "an extreme red line," he was quoted as saying by the state-controlled media on Thursday.
The spokesman also criticized the statement by US Secretary of Defense Lloyd Austin this week in Seoul that US strategic weapon systems such as fighter jets or aircraft carriers would again be sent to the peninsula.
North Korea will show the "strongest response" according to the principle of "nuclear weapons against nuclear weapons and total confrontation against total confrontation".
The conflict with North Korea has intensified considerably since last year.
The country, ruled autocratically by Kim Jong Un, increased the scope and speed of its tests with nuclear-capable missiles and intensified its rhetoric, despite a ban by UN resolutions.
South Korea and the US resumed full-scale military exercises.
Austin and his South Korean colleague Lee Jong Sup announced in Seoul on Tuesday that they would expand their joint maneuvers.
The aim is to strengthen the alliance's capabilities, including information sharing and joint planning.