North Korea threatened the United States on Thursday with a preemptive nuclear strike, raising the level of rhetoric while the UN Security Council considers 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 said it will scrap the armistice with Washington that ended hostilities in the 1950-53 Korean War.

"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 UN 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 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 as invalid.

It is the latest in an escalation of tough words from both sides of the armed Korean border this week as the UN Security Council deliberates a resolution to tighten financial sanctions and a naval blockade against the North.

North Korea has protested against the UN censures of its rocket launches, which it says are part of peaceful space program, as an exercise of double standard masterminded by the United States.

But North Korea's shrill rhetoric rarely goes beyond just that. Its latest armed aggression against the South in 2010 came unannounced, bombing a South Korean island killing two civilians. It is widely accused of sinking a South Korean navy ship earlier in the year, killing 46 sailors.

North Korea is conducting a series of military drills and is getting ready for state-wide war practice of an unusual scale, South Korea's defense ministry said earlier on Thursday.

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 exercise at the state level," South Korea's defense 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.

In the latest threat coming under its new young leader, Kim Jong-un, a top North Korean general on Tuesday said 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.