North Korea upped its rhetoric on Thursday, vowing to enlarge its atomic arsenal and warning of a "fire shower of nuclear retaliation" in the event of a US attack. The jingoist statements come as the regime marked the 1950 outbreak of the Korean War. Meanwhile, the US Navy followed a North Korean ship suspected of carrying weapons in violation of a UN resolution punishing Pyongyang's May 25 nuclear test, and as anticipation mounted that the North might test-fire short- or mid-range missiles in the coming days. President Barack Obama extended US economic sanctions against North Korea for another year Wednesday, saying the North's possession of "weapons-usable fissile material" and its proliferation risk "continue to pose an unusual and extraordinary threat" to the United States, according to the White House Web site. According to the 2008 document outlining the restrictions, "all property and interests in property of North Korea or a North Korean national ... were blocked." The US measures are on top of UN sanctions that bar member states from buying weapons from or selling them to North Korea. They also ban the sale of luxury goods to the isolated country and prohibit the provision of weapons-related technical training and financial transactions. State-run newspapers in Pyongyang ran lengthy editorials accusing the US of invading the country in 1950 and of looking for an opportunity to attack again. The editorials said those actions justified North Korea's development of atomic bombs to defend itself. The North "will never give up its nuclear deterrent ... and will further strengthen it" as long as Washington remains hostile, Pyongyang's main Rodong Sinmun newspaper said. In a separate commentary, the paper blasted a recent US pledge to defend South Korea with its nuclear weapons, saying that amounted to "asking for the calamitous situation of having a fire shower of nuclear retaliation all over South Korea." Historical evidence shows it was North Korea that started the Korean War by invading the South, but Pyongyang claims the U.S. was to blame. The totalitarian government apparently hopes to infuse North Koreans with fear of a fresh American attack to better control the hunger-stricken population. The US fought alongside the South, leading UN forces, during the war. The conflict ended in 1953 with a truce, not a peace treaty, leaving the peninsula divided and in a state of war. The US has 28,500 troops in South Korea to protect against renewed hostilities. The US has repeatedly said it has no intention of attacking the North. The new UN resolution seeks to clamp down on North Korea's trading of banned arms and weapons-related material by requiring UN member states to request inspections of ships carrying suspicious cargo. North Korea has said it would consider any interception of its ships a declaration of war. Adding to the tensions, North Korea has been holding two US journalists since March. The reporters, Laura Ling and Euna Lee, were sentenced to 12 years of hard labor for illegal border crossing and hostile acts earlier this month. Ling's husband, Iain Clayton, said Wednesday that his wife called him on Sunday night and she sounded scared. He also said Ling's medical condition has deteriorated and Lee has developed a medical problem. Ling reportedly suffers from an ulcer. The Kang Nam - the ship the US is tracking - is the first North Korean ship to be followed under the resolution. It left the North Korean port of Nampo a week ago and is believed bound for Myanmar, South Korean and US officials said. A senior US defense official said Wednesday that the ship had already cleared the Taiwan Strait. He said he didn't know how much range the Kang Nam has - whether or when it may need to stop at a port to refuel - but that the ship has in the past stopped in Hong Kong. Another US defense official said he tended to doubt reports that the Kang Nam was carrying nuclear-related equipment, saying the information officials had received seemed to indicate the cargo was conventional munitions. The US officials spoke on condition of anonymity because they were discussing intelligence. The US and its allies have not decided whether to contact and request an inspection of the ship, Pentagon press secretary Geoff Morrell said Wednesday. Reports about possible missile launches from the North highlighted the tension on the Korean peninsula. The North has designated a no-sail zone off its east coast from June 25 to July 10 for military drills. A senior South Korean government official said the ban is believed connected to North Korean plans to fire short- or mid-range missiles. He spoke on condition of anonymity, citing department policy. Meanwhile, South Korea is trying to organize talks among North Korea's five negotiating partners in six-nation nuclear talks - the US, China, Japan, South Korea and Russia - in an attempt to get Pyongyang back to the negotiating table.