Tens of thousands of North Koreans shouted slogans to denounce international sanctions at a rally in central Pyongyang on Thursday, as the communist country vowed to enlarge its atomic arsenal and warned of a "fire shower of nuclear retaliation" in the event of a U.S. attack.

The rally to mark the anniversary of the 1950 outbreak of the Korean War came a day after President Barack Obama extended U.S. economic sanctions against North Korea for another year, 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.

The U.S. measures are on top of U.N. sanctions imposed on the North over its nuclear test in May. The U.N. sanctions 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 financial transactions.

In Pyongyang, an estimated 100,000 packed the main square, shouting "Let's smash!" in unison while punching clenched fists in the air, footage from APTN in North Korea showed. A placard showed hands crushing a missile with "U.S." written on it.

The isolated, totalitarian regime often organizes such massive rallies at times of tension with the outside world.

North Korea's "armed forces will deal an annihilating blow that is unpredictable and unavoidable, to any 'sanctions' or provocations by the US," Pak Pyong Jong, first vice chairman of the Pyongyang City People's Committee, told the crowd.

State-run newspapers ran lengthy editorials accusing the U.S. 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.

The new U.N. resolution — passed to punish Pyongyang after its May 25 nuclear test — seeks to clamp down on North Korea's trading of banned arms and weapons-related material by requiring U.N. 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.

The U.S. Navy is currently following a North Korean ship suspected of carrying weapons in violation of the resolution, but Pentagon press secretary Geoff Morrell said Wednesday that the U.S. and its allies have not decided whether to contact and request an inspection of the ship.

The Kang Nam left the North Korean port of Nampo a week ago and is believed bound for Myanmar, South Korean and U.S. officials have said. A senior U.S. defense official said Wednesday that the ship had already cleared the Taiwan Strait.

Another U.S. 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 U.S. officials spoke on condition of anonymity because they were discussing intelligence.

Adding to the tensions, anticipation is mounting that the North might test-fire short- or mid-range missiles in the coming days. 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.

The North has also been holding two U.S. 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.