North Korea on Wednesday barred South Korean workers from entering a jointly run factory park just over the heavily armed border in the North, officials in Seoul said, a day after Pyongyang announced it would restart its long-shuttered plutonium reactor and increase production of nuclear weapons material.

North Korea said Tuesday it will escalate production of nuclear weapons material, including restarting a long-shuttered plutonium reactor, in what outsiders see as Pyongyang's latest attempt to extract U.S. concessions by raising fears of war.

South Korea's new president on Monday promised a strong military response to any North Korean provocation after Pyongyang announced that the two countries were now in a state of war.

The United States sent F-22 stealth fighter jets to South Korea on Sunday to join military drills aimed at underscoring the U.S. commitment to defend Seoul in the face of an intensifying campaign of threats from North Korea.

North Korea named former premier Pak Pong Ju, a key confidant of the leadership dynasty who was sacked in 2007 for failing to implement economic reforms, as its cabinet chief on Monday in a move that would further cement the ruling family's grip on power.

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.