"From this moment, the Supreme Command puts all of its field artillery including strategic rocket units and long-range artillery units into the No. 1 combat ready posture," the Korean People's Army's Supreme Command said in a statement carried by the country's Korean Central News Agency.
The statement said artillery units were targeted on South Korea, as well as the U.S. mainland, Hawaii, Guam and U.S. military installations in the Pacific, South Korea's Yonhap news agency reported.
In response, U.S. military officials said the U.S.-South Korea alliance won't be bullied by North Korea's announcement.
"North Korea's bellicose rhetoric and threats follow a pattern designed to raise tensions and intimidate others," the U.S. Forces-Korea said in a statement.
"While the United Nations Command, Combined Forces Command and U.S. Forces Korea continue to maintain the armistice, North Korea continues its provocative behavior and rhetoric," the statement said. "The Republic of Korea-U.S. alliance is strong and we remain committed to the defense of [South Korea]."
North Korea said that, in its "crystal clear judgment," it cannot ignore the United States' nuclear threats and military actions anymore, Yonhap reported.
North Korea will demonstrate "our army and people's stern reaction to safeguard our sovereignty and the highest dignity [of leader Kim Jong Un] through military actions," the statement said.
In its latest threat, North Korea said South Korea should be "mindful that everything will be reduced to ashes and flames the moment the first attack is unleashed."
In Seoul, the Defense Ministry said it was closely monitoring the communist country, looking for any signs of provocations.
South Korea had raised its military readiness and surveillance status earlier in March during joint drills with U.S. forces – which drew an angry response from Pyongyang -- and has maintained the heightened alert status since.
"The No. 1 combat readiness status is a term first used by North's state media, which means the highest level of combat readiness status," a senior Defense Ministry official told Yonhap. "It seems to be aimed at creating a war-like situation to unite the North Korean people, though it could result in real provocations."
Tensions on the Korean Peninsula have increased after North Korea launched a three-stage rocket in December and conducted its third nuclear test last month. In response, the U.N. Security Council passed another resolution imposing more sanctions on North Korea. Earlier this month, the United States and South Korea began their annual joint military drills aimed at warning North Korea against attacking its southern neighbor.
North Korea, angered over the sanctions and joint drills, has been issuing highly provocative threats, including cutting off the Red Cross telephone line between Pyongyang and Seoul, and unilaterally scrapping the 1953 Korean Armistice that ended the 1950-53 Korean War.