South Korea ? South Korea’s president said today that 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 U.N. 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 U.S. 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.