Seoul: North Korea on Wednesday warned of a "thousand-fold" military retaliation against the US and its allies if provoked, the latest threat in a drumbeat of rhetoric in defence of its rogue nuclear programme.

Japanese and South Korean news reports said North Korea is preparing an additional site for test-firing a long-range missile capable of striking the US

The warning of a military strike, carried by the North's state media, came hours after President Barack Obama declared North Korea a "grave threat" to the world, and pledged that recent UN sanctions on the communist regime will be aggressively enforced.

Obama and South Korean President Lee Myung-bak met in Washington on Tuesday for a landmark summit in which the two leaders agreed to build a regional and global "strategic alliance" to persuade North Korea to dismantle all its nuclear weapons.

Pyongyang claims its nuclear bombs are a deterrence against the United States and accuses Washington of plotting with Seoul to topple its secretive regime led by the unpredictable dictator Kim Jong Il who is reportedly preparing to hand over power to his 26-year-old son.

"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," the government-run Minju Joson newspaper said.

The commentary, carried by the official Korean Central News Agency, also called Obama "a hypocrite" for advocating a nuclear-free world while making "frantic efforts" to develop new nuclear weapons at home.

"The nuclear programme is not the monopoly of the US," it said without mentioning the Obama-Lee summit.

Attention has been focused on North Korea since it conducted its second nuclear rest on May 25 in defiance of the United Nations.