The country's parliament approved the appointment of a new premier, Pak Pong Ju, who is seen by outside experts as an economic reformer. This comes one day after top party officials adopted new top priorities, including nuclear arms and a stronger economy.
Un's regime declared Sunday that nuclear weapons are "the nation's life" and vowed not to trade them for "billions of dollars."
Pyongyang has issued near-daily threats, including vows of nuclear strikes on South Korea and the U.S. Pyongyang state media on Saturday declared the country is a "state of war" with South Korea.
The U.S. is taking those threats seriously, deploying two F-22 stealth fighter jets Sunday from Japan over South Korea in annual U.S.-South Korean war games that Pyongyang calls preparation for an invasion.
"We believe that this is part of their overheated rhetoric and something that we think they should avoid," said George Little, Pentagon spokesman. "We have a range of options to counter their provocations and threats. obviously we hope to never have to put those plans into effect."
The new South Korean president, who has a policy meant to re-engage Pyongyang with talks and aid, told her top military leaders Monday to set aside political considerations and respond strongly should North Korea attack.
Analysts say the rising tension could spark a military clash on the Korean peninsula, though full-blown war is unlikely.
The U.S. last week took the unprecedented step of sending two nuclear-capable B-2 bombers from Missouri to South Korea and back.
Still, Pentagon officials insist the U.S. and North Korea are not at war.
The Associated Press contributed to this report.