SEOUL (AFP) - North Korea said Saturday it has started reprocessing spent fuel rods to make weapons-grade plutonium, in an apparent response to international punishment against its controversial rocket launch. The statement came hours after the United Nations slapped sanctions on three North Korean firms accused of backing missile development, in its first concrete action against Pyongyang over the April 5 rocket launch. The reprocessing of spent fuel rods from the pilot atomic power plant began as declared in the Foreign Ministry statement dated April 14, a Foreign Ministry spokesman told the official Korean Central News Agency. This will contribute to bolstering the nuclear deterrence for self-defence in every way to cope with the increasing military threats from the hostile forces. North Korea on April 14 announced it would quit six-nation nuclear disarmament talks and restart its atomic weapons programme in protest at the UNs statement condemning the launch. The North had been disabling parts of the Yongbyon nuclear complex as agreed under a February 2007 six-nation deal involving the two Koreas, the United States, China, Russia and Japan. But six-party negotiations stalled last December because of disputes about ways to verify its declared nuclear activities. Pak Tok-Hun, North Koreas deputy permanent representative to the United Nations, denounced the curbs on three Korean firms as a wanton violation of the United Nations charter. It is the inalienable right of every nation and country to make peaceful use of outer space, Pak said in reference to the April 5 rocket launch. Russian Foreign Minister Sergei Lavrov, meeting with President Lee Myung-Bak in Seoul Saturday after a visit to Pyongyang, reaffirmed Moscows stance against sanctions. North Korea is now like a fortress under siege. Other countries must not react too emotionally (to the rocket launch), Lavrov was quoted as telling Lee by a South Korean official. North Korea, marking the anniversary of its military Saturday, announced it would deal a merciless strike against the US and its allies should they try to invade. Should the imperialist US and its followers touch even an inch of our territorial land, air or sea, our troops of the Korean Peoples Army will deal a merciless strike of justice to the enemies to destroy them and thoroughly eradicate the roots of war on the Korean Peninsula, the communist party newspaper Rodong Sinmun said. Meanwhile, the US on Saturday blasted Pyongyangs decision to reactivate its nuclear programme and called for the reclusive nation to return to international denuclearisation talks. We will not accept North Korea as a nuclear-weapons state, said Megan Mattson, a State Department spokeswoman.