A senior North Korean diplomat has accused the US of "crossing the red line" and effectively declaring war by putting leader Kim Jong Un on its list of sanctioned individuals.
Han Song Ryol, director-general of the US affairs department at Pyongyang's Foreign Ministry, said recent actions by Washington have put the situation on the Korean peninsula on a war footing.
He also said a vicious showdown could erupt if the US and South Korea hold annual war games as planned next month.
The two allies regularly conduct joint military exercises south of the demilitarised zone, and Pyongyang typically responds with tough talk and threats of retaliation.
Mr Han said North Korea believes the manoeuvres have become openly aggressive because they reportedly now include training designed to prepare troops for the invasion of the North's capital and "decapitation strikes" aimed at killing its leadership.
Mr Han said designating Kim himself for sanctions was the final straw.
"The Obama administration went so far to have the impudence to challenge the supreme dignity of the DPRK (Democratic People's Republic of Korea) in order to get rid of its unfavourable position during the political and military showdown with the DPRK," he said.
"The United States has crossed the red line in our showdown. We regard this thrice-cursed crime as a declaration of war."
Although North Korea had already been heavily sanctioned internationally for its nuclear weapons and long-range missile development programmes, Washington's announcement on July 6 was the first time Kim had been personally sanctioned.
Less than a week later, Pyongyang cut off its final official means of communications with Washington - known as the New York channel. Mr Han said the North has made it clear that everything between the two must now be dealt with under "war law".
Kim and 10 others were put on the list of sanctioned individuals in connection with alleged human rights abuses, documented by the United Nations Human Rights Commission, including a network of political prisons and harsh treatment of any kind of political dissent in the authoritarian state.
US State Department officials said the sanctions were intended in part to highlight those responsible for the abuses and to pressure lower-ranking officials to think twice before carrying them out.