A photo released 22 May 2009 of Kim Jong Il(L) watching an Air Force flight training at an unidentified location in N. Korea

The United States said Monday that a scientific analysis of North Korea's May 25 nuclear test shows it to have had only a fraction of the explosive force first estimated. The U.S. intelligence report came as President Barack Obama prepared for talks Tuesday on the North Korean nuclear program and other issues with South Korean President Lee Myung-bak.  

A brief statement from U.S. Director of National Intelligence Dennis Blair said North Korea probably conducted an underground nuclear test on May 25 that had an explosive yield of a few kilotons, the equivalent of a few thousand tons of TNT.

Although it was a sizable explosion, the test was small by nuclear weapon standards and only a fraction of the 10 to 20 kiloton estimate by Russia's Defense Ministry issued a few days after the event.

By contrast, North Korea's first nuclear test, in November 2006, was estimated at just one kiloton and is considered by some experts to have been a partial failure.

Although the statement by the U.S. intelligence chief spoke of a "probable" nuclear test, officials say they do not have serious doubts that the May 25 detonation was nuclear, given the difficulty and cost of simulating a blast of that magnitude with conventional explosives.

The U.N. Security Council voted unanimously last Friday to tighten financial and other sanctions against North Korea because of the test - actions Pyongyang responded to defiantly with vows to build more plutonium bombs and start a new weapons program based on uranium enrichment.

At a news briefing on Monday, State Department Spokesman Ian Kelly called on Pyongyang to end its bellicose threats and return to Chinese-sponsored six-party negotiations on its nuclear program.

"We just passed an extremely tough resolution on Friday that showed incredible unity among the Security Council and with South Korea and Japan," said Ian Kelly. "We're going to be focused on implementing that resolution. And beyond that, North Korea knows what it has to do. North Korea needs to give up all this, all this rhetoric and belligerent actions, and return to the six-party talks unconditionally."

South Korean President Lee Myung-bak (file photo)

North Korea agreed in principle in 2005 to give up its nuclear program in return for aid and diplomatic benefits. But the six-party talks stalled last year and Pyongyang withdrew from them earlier this year after U.N. criticism of a long-range missile test North Korea says was a satellite launch.

South Korean President Lee Myung-bak arrived in Washington on Monday and held a late afternoon meeting with Secretary of State Hillary Clinton in preparation for his White House meeting with President Obama on Tuesday - both of which are expected to be dominated by the nuclear issue.

Before leaving Seoul, the South Korean leader said that despite recent North Korean rhetoric, the danger of a regional nuclear conflict is remote. He said the real threat is that North Korean nuclear technology might be sold to rogue states or terrorist groups.