Seoul — North Korea vowed on Monday to bolster its nuclear arsenal and launch "thousands-fold" revenge against the United States to respond to tough UN sanctions imposed after its intercontinental ballistic launches.The North's warning came two days after the UN Security Council unanimously approved new sanctions to punish the North including a ban on coal and other exports worth over $1bn. US Ambassador Nikki Haley called the US-drafted resolution "the single largest economic sanctions package ever levelled against" North Korea.In a statement carried by state media, the North Korean government said the sanctions were a "violent infringement of its sovereignty" that was caused by a "heinous US plot to isolate and stifle" North Korea.It said the UN sanctions will never force the country to negotiate over its nuclear programme or to give up its push to strengthen its nuclear capability. The North said it will take "action of justice", but didn't elaborate.North Korea test-launched two ICBMs last month as part of its efforts to possess a long-range missile capable of striking anywhere in the mainland US. Both missiles were fired at highly lofted angles and analysts say the weapons are capable of reaching parts of the United States if fired at a normal, flattened trajectory.The centrepiece of the UN sanctions is a ban on North Korea exports of coal, iron, lead and seafood products — and a ban on all countries importing these products, estimated to be worth over $1bn in hard currency.According to a Security Council diplomat, coal has been North Korea's largest export, earning $1.2bn last year which was then restricted by the Security Council in November to a maximum $400m. This year, Pyongyang was estimated to earn $251m from iron and iron ore exports, $113m from lead and lead ore exports, and $295m from fish and seafood exports, the diplomat said.The council diplomat was not authorised to speak publicly and insisted on anonymity.