SEOUL, South Korea (AP) – South Korea's President Lee Myung-bak said Monday the country's alliance with the United States is key to resolving North Korea's nuclear and missile threats as he flew to Washington for a summit with President Barack Obama.

The summit scheduled for Tuesday comes in the wake of North Korea's weekend declaration that it would step up its nuclear bomb-making program. It also threatened war with any country that tries to stop its ships on the high seas as part of new United Nations Security Council sanctions passed in response to Pyongyang's May 25 nuclear test.

North Korea is believed to have enough weaponized plutonium for at least half a dozen atomic bombs, and a US government official said last week Pyongyang may be preparing for another nuclear test, its third. The official spoke on condition of anonymity in order to discuss the unreleased information and provided no details. US and South Korean intelligence were keeping a close eye on signs of an impending test.

"We cannot stress enough the importance of diplomacy at a time when a security crisis is intensifying due to North Korea's nuclear and missile threats," President Lee Myung-bak said in a radio speech before his departure Monday.

"In particular, the South Korea-US diplomacy is key to that diplomacy," he said. "I will use this summit to reconfirm the strong Korea-US alliance."

The strong ties between South Korea and the United States are a thorn in the side of wartime foe North Korea, which accuses the two countries of plotting an attack and a desire to topple the communist regime.

The two Koreas technically remain at war because their three-year conflict ended in a truce, not a peace treaty, in 1953, and they remain divided by a heavily fortified border.

The US has 28,500 troops in South Korea but denies any plan to attack North Korea.

However, Washington fears that North Korea, one of the poorest nations in the world, will sell its nuclear technology to rogue nations, spreading the atomic threat.