SEOUL, South Korea — A supercarrier sent jets into overcast skies today in U.S.-South Korean military drills that North Korea warned could spark war, but signs of diplomacy emerged alongside the tensions over last week’s deadly North Korean attack.

The North’s only major ally, China, hosted a top North Korean official for talks, and Japan also planned to send an envoy to China. The U.S., South Korea and Japan agreed to talk next week in Washington about the North’s nuclear weapons and its Nov. 23 artillery barrage that killed four South Koreans.

It was unclear if the Beijing visit by North Korea’s Choe Thae Bok, chairman of the North’s Parliament, would lead to any diplomatic solution. China, under pressure to rein in its ally, proposed emergency regional talks earlier this week, but South Korea, the United States and Japan gave a cool response.

Even as diplomats scrambled, leaked U.S. diplomatic cables revealed signs of a rift in the relationship between China and North Korea, a striking contrast from official statements underscoring their strong historical ties.

Documents from the whistle-blowing website WikiLeaks showed China’s frustration with the North and speculated that Beijing would accept a future Korean peninsula unified under South Korean rule.

The North, meanwhile, reminded the world it was forging ahead with its nuclear efforts. Pyongyang said today that it’s operating a modern uranium enrichment plant equipped with thousands of centrifuges.

The North first revealed the uranium program in early November to a visiting American scientist.