North Korea's Feb. 12 nuclear test and continued military threats have sparked calls for Seoul to take a tougher stance against the secluded regime including enhanced nuclear deterrence.

North Korea vowed Sunday to strengthen its nuclear weapons, a day after announcing it is in a “state of war” with South Korea, and said it would never trade its atomic deterrent for aid.

The outside world focuses on the messages of doom and gloom from North Korea: bombastic threats of nuclear war, fantasy videos of U.S. cities in flames, digitally altered photos of leader Kim Jong Un guiding military drills. But back home, North Koreans get a decidedly softer dose of propaganda: Kim portrayed as a young, energetic leader, a people person and family man.

North Korea said on Saturday it was entering a "state of war" with South Korea in a continuing escalation of angry rhetoric directed at Seoul and Washington, but the South brushed off the statement as little more than tough talk.

North Korean leader Kim Jong Un on Friday ordered missile units to prepare to strike U.S. mainland and military bases, vowing to “settle accounts” after U.S. stealth bombers flew over South Korea.

North Korean state media issued two photos Friday that, either by accident or design, appeared to show plans for striking the U.S. mainland, as well as details of the North's military strength.

North Korea put its missile units on standby on Friday to attack U.S. military bases in South Korea and the Pacific, after the United States flew two nuclear-capable stealth bombers over the Korean peninsula in a rare show of force.

The United States said two nuclear-capable B-2 stealth bombers flew “deterrence” missions over South Korea on Thursday, defying apocalyptic threats of retribution from North Korea against ongoing war games.

A day after shutting down a key military hotline, Pyongyang instead used indirect communications with Seoul to allow South Koreans to cross the heavily armed border and work at a factory complex that is the last major symbol of inter-Korean cooperation.

A heavily armed border crossing between North and South Korea that allows the North access to $2 billion in trade a year, one of its few avenues to foreign currency, remained open on Thursday despite Pyongyang's move to cut communications.