(Adds U.N. resolution adopted, U.S. envoy comments)
By Jack Kim and Louis Charbonneau
SEOUL/UNITED NATIONS, March 7 (Reuters) - North Korea threatened the United States on Thursday with a preemptive nuclear strike, raising the level of rhetoric just before the U.N. Security Council approved new sanctions against the reclusive country.
North Korea has accused the United States of using military drills in South Korea as a launch pad for a nuclear war and has scrapped the armistice with Washington that ended hostilities in the 1950-53 Korean War.
North Korea, which has one major ally, neighbouring China, threatens the United States and its "puppet", South Korea, on an almost daily basis.
"Since the United States is about to ignite a nuclear war, we will be exercising our right to preemptive nuclear attack against the headquarters of the aggressor in order to protect our supreme interest," the North's foreign ministry spokesman said in a statement carried by the official KCNA news agency.
North Korea conducted a third nuclear test on Feb. 12, in defiance of U.N. resolutions, and declared it had achieved progress in securing a functioning atomic arsenal. It is widely believed the North does not have the capacity to deliver a nuclear strike on the mainland United States.
The U.N. Security Council voted on Thursday to tighten financial restrictions on Pyongyang and crack down on its attempts to ship and receive banned cargo in breach of U.N. sanctions.
The new sanctions were agreed after three weeks of negotiations between the United States and China, which has a history of resisting tough measures on its neighbour and ally.
U.N. Secretary-General Ban Ki-moon, a former South Korean foreign minister, welcomed the council's move, saying in a statement that the resolution "sent an unequivocal message to (North Korea) that the international community will not tolerate its pursuit of nuclear weapons."
North Korea's latest threats were the latest in an escalation of tough words from both sides of the armed Korean border this week.
The North's unnamed foreign ministry spokesman also said it would be entitled to take military action as of March 11 when U.S.-South Korea military drills move into a full-scale phase as it had declared the truce invalid.
President Barack Obama's administration said it had reassured South Korea and Japan "at the highest levels" of its commitment to deterrence, through the U.S. nuclear umbrella and missile defense, in the face of fresh threats from North Korea.
Glyn Davies, the State Department's point man for North Korea policy, also said in testimony prepared for a Senate hearing that Washington will not accept North Korea as a nuclear state.
North Korea, which held a mass military rally in Pyongyang on Thursday in support of its recent threats, has protested against the U.N. censures of its rocket launches. It says they are part of a peaceful space programme and that the criticism is an exercise of double standards by the United States.
The North's shrill rhetoric, however, rarely goes beyond just that. Its last armed aggression against the South in 2010 came unannounced, bombing a South Korean island killing two civilians. It was widely accused of sinking a South Korean navy ship earlier in the year, killing 46 sailors.
North Korea was conducting a series of military drills and getting ready for state-wide war practice of an unusual scale, South Korea's defense ministry said earlier.
South Korea and the United States, which are conducting annual military drills until the end of April, are watching the North's activities for signs they turn from an exercise to an actual attack, a South Korean official said.
"It hasn't been frequent that the North conducted military exercises at the state level," South Korea's defence ministry spokesman, Kim Min-seok, said. "The North is currently conducting various drills on land, at sea and aerially.
"We are watching the North's activities and stepping up readiness under the assumption that these drills can lead to provocation at any time."
Kim declined to confirm news reports that the North has imposed no-fly zones off its coasts in a possible move to fire missiles, but he said any flight ban limited to near the coast would not be for weapons with meaningful ranges.
A North Korean general said on Tuesday that Pyongyang was scrapping the armistice. But the two sides remain technically at war as the civil war did not end with a treaty.
South Korea's military said in a rare warning on Wednesday that it would strike back at the North and target its leadership if Pyongyang launched an attack. (Additional reporting by Paul Eckert in Washington; Writing by Claudia Parsons; editing by Nick Macfie and Christopher Wilson)