North Korea warned South Korea and the United States on Wednesday that Seoul's participation in a US-led program to intercept ships suspected of carrying weapons of mass destruction is equal to a declaration of war. South Korea announced its participation in the US-led program on Tuesday, one day after North Korea defiantly conducted a nuclear test, drawing international criticism. The North's military said in a statement that it will respond with "immediate, strong military measures" against any attempt to stop and search North Korean ships under the Proliferation Security Initiative. The statement, carried by the North's official Korean Central News Agency, also said the regime no longer considers itself bound by the armistice that ended the Korean War. It accused the US, a signatory of the armistice, of "dragging" the South into the program under its "hostile policy" against the North. It also said it cannot guarantee safety for South Korean and US navy ships sailing near the disputed western Korean sea border. Earlier Wednesday, news reports and South Korean officials said the North has restarted a weapons-grade nuclear plant and fired five short-range missiles in two days, deepening the standoff with world powers following its nuclear test. South Korea's mass-circulation Chosun Ilbo newspaper reported that US spy satellites have detected steam coming from a nuclear facility at North Korea's main Yongbyon plant, indicating the North is reprocessing spent nuclear fuel rods to harvest weapons-grade plutonium. Its report quoted an unnamed official. South Korea's Defense Ministry and the National Intelligence Service - the country's main spy agency - said they cannot confirm the report. The North had said it would begin reprocessing in protest over international criticism of its April 5 rocket launch. North Korea is believed to have enough plutonium for at least half a dozen atomic bombs. The North also has about 8,000 spent fuel rods which, if reprocessed, could allow the country to harvest 6-8 kilograms of plutonium - enough to make at least one nuclear bomb, experts said. Yonhap news agency carried a similar report later Wednesday, saying the gate of a facility storing the spent fuel rods was spotted open several times since mid-April. The report, also citing an unnamed South Korean official, said chemical-carrying vehicles were spotted at Yongbyon. North Korea test-fired three additional short-range missiles Tuesday, including one late at night, from the east coast city of Hamhung, according to South Korean Defense Ministry spokesman Won Tae-jae. He said the North already test-launched two short-range missiles from another eastern coast launch pad on Monday, not the three reported by many South Korean media outlets.