North Korea will have a limited ability to deliver a weapon to U.S. shores within five years using intercontinental ballistic missiles, Gates predicted. North Korea has threatened to test such missiles, and has already conducted underground nuclear tests that prove it has manufactured at least rudimentary nuclear weapons.
"With the North Koreans' continuing development of nuclear weapons and their development of intercontinental ballistic missiles, North Korea is becoming a direct threat to the United States, and we have to take that into account," Gates said.
The risk of war on the Korean Peninsula is also rising because South Koreans are fed up with provocation and harassment from the North, Gates said.
"We consider this a situation of real concern and we think there is some urgency to proceeding down the track of negotiations and engagement," he said.
North Korea is accused of sinking a South Korean Navy ship last spring, killing 46 sailors, and it fired artillery at a disputed island in November, killing four South Koreans.
The South's "tolerance for not responding" is nearly gone, Gates told reporters in China, which is North Korea's only ally.
"Clearly, if there is another provocation there will be pressure on the ... South Korean government to react," Gates said.
Gates said he thanked Chinese President Hu Jintao and others he saw here for reining in North Korea, and asked China to keep leaning on the fellow communist state.
North Korea depends on China for aid and protection, but China's influence over the inward-looking nation is limited. China props up the insolvent North largely out of fear that a collapsed state would unsettle the entire North Asian region.
U.S. officials have said North Korea's increasingly bellicose behavior over the past year is probably part of a plan to establish the military bona fides of leader Kim Jong Il's son as his chosen successor. North Korea regularly denounces the United States and accuses it of wanting to destroy the country, but it poses the most direct threat to its neighbor South Korea, a U.S. ally.
War on the Korean peninsula could involve the United States apart from any North Korean ability to deliver a nuclear weapon across the Pacific. The United States has 29,000 troops stationed in South Korea as a legacy of the Korean War, and is pledged to help defend Seoul.
Gates will visit Seoul on Friday for talks on the North Korean threat.
North Korea has pleaded for talks in recent days, and has proposed holding a working-level dialogue on Jan. 27 to prepare for higher-level government discussions and Red Cross talks on joint economic projects on Feb. 1.
South Korea's Unification Ministry has rejected the North's latest offer as an attempt to win economic aid.
"If the South Korean authorities sincerely want the improvement of North-South relations, they should clear away useless doubt and open the door of their minds and actively respond to our proposal for dialogue and goodwill measures," Min Kum Song, a North Korean official, told Associated Press Television News in Pyongyang on Tuesday.
Gates said he wants to see North Korea take specific steps, such as a moratorium on nuclear and missile tests, to show that it is serious about disarmament talks.
Gates said he wants to make sure North Korea is not rewarded for brinksmanship. He said the North has a pattern of bargaining by provoking a crisis, "and then everybody scrambles diplomatically to try and put Humpty Dumpty back together again. I don't want to buy the same horse twice."