North Korea has been angered by tougher UN sanctions
North Korea has threatened a "thousand-fold" military retaliation against the US and its allies if it is provoked.
The warning, carried by state media, came after US President Barack Obama said that a nuclear-armed North Korea posed a "grave threat" to the world.
At a news conference with South Korean President Lee Myung-bak, Mr Obama said the US would "vigorously" pursue an end to North Korea's nuclear programme.
North Korea has recently conducted nuclear and missile tests.
"If the US and its followers infringe upon our republic's sovereignty even a bit, our military and people will launch a one hundred- or one thousand-fold retaliation with merciless military strike," said a commentary published by state news agency KCNA.
"The nuclear programme is not the monopoly of the US," it said.
The commentary is the latest threat from Pyongyang as tensions in the region have escalated over its 25 May underground nuclear test and recent missile launches.
Japanese and South Korean media reported that there were signs of two long-range missile launch sites being readied - one on the north-west coast and the other on the north-east coast.
It was previously believed a launch might come from the north-west site, not far from the Chinese border.
President Obama: "We have continually insisted that North Korea denuclearize'
North Korea is also withdrawing funds from its bank accounts in the Chinese territory of Macau and elsewhere before they can be frozen by new UN sanctions, according to South Korea's Dong-a Ilbo newspaper.
On 12 June the UN Security Council approved tougher sanctions against North Korea, including inspections of ships suspected of taking banned cargo to and from North Korea, a wider ban on arms sales and further measures to cut Pyongyang's access to international financial services.
Following the resolution, the North said it would start enriching uranium and use all its plutonium for nuclear weapons.
Japan has taken action by banning all trade with North Korea. Pyongyang's main ally, China, said it would "earnestly implement" the new sanctions.
At a summit in Washington on Tuesday, Mr Obama said that he and his South Korean counterpart had agreed that a new UN resolution designed to halt North Korea's nuclear ambitions should be fully enforced.
"Under no circumstance are we going to allow North Korea to possess nuclear weapons," said Mr Lee.
And he pledged to end a cycle of letting North Korea create a crisis in order to be rewarded with concessions from the international community.
"This is a pattern they've come to expect," Mr Obama said.
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