BEIJING — China’s president urged North Korea yesterday to refrain from increasing tensions over its nuclear program and to return to disarmament talks as diplomats worked to forestall U.N. sanctions against the regime.

America’s top nuclear envoy made an unscheduled trip to China,saying efforts to resolve the crisis have reached a crucial point. A delegation from North Korea also came to Beijing.

China’s Foreign Ministry criticized a Japanese proposal that demands the North stop developing, testing, and selling ballistic missiles as “an overreaction.”

Cabinet-level talks between North and South Korea, meanwhile, kicked off with the South saying Pyongyang’s missile tests were destabilizing the region.

President Hu of China told the visiting vice president of the North’s parliament, Yang Hyong Sop: “We are against any actions that will aggravate the situation. We hope that relevant parties will do more things conducive to the peace and stability of the peninsula,” according to the official Xinhua News Agency.

Mr. Hu said Beijing is “seriously concerned” and called for progress in stalled six-nation talks over the North’s nuclear program.

The warning by Mr. Hu, who rarely speaks publicly about North Korea, represented an unusually firm stance by Beijing and appeared to reflect growing frustration with its ally.