Kerry, in his most strident call yet for direct talks with Pyongyang, said Bush had sat by and allowed Pyongyang's arsenal to rise from two nuclear weapons to six or eight bombs.

"That means that North Korea ... a country that will sell anything to anyone, can sell nuclear weapons and still hold and arsenal in reserve," said Kerry in a major speech on national security in Iowa.

"I will work with our allies to get the six party talks back on track. And I will talk directly to the North Koreans, as our South Korean, Chinese and Russian partners have requested us to do."

Bush's predecessor Bill Clinton did conduct one on one talks with North Korea, sending his Secretary of State Madeleine Albright to Pyongyang to meet North Korean leader Kim Jong-Il in late 2000.

But Bush halted that approach when he came to office, a position solidified when it emerged that North Korea had been enriching uranium, in violation of a 1994 anti-nuclear treaty with Washington.

The current administration says bilateral talks with Pyongyang would be tantamount to offering a reward to the Stalinist state for bad behavior.

It has addressed the crisis through six-party talks involving Washington and Pyongyang, but also China, South Korea, Japan and Russia -- though there has been little progress.