U.S. Secretary of State John Kerry was in the region last weekend to address the Korean crisis. He said he was willing to tackle concerns through multilateral talks that include Chinese and North Korean representatives.
"But Pyongyang must prove its seriousness by taking meaningful steps to abide by its international obligations," a U.S. State Department statement said.
South Korean Foreign Minister Yun Byung-se said China was playing an effective role in mediating the crisis.
"As long as North Korea continues to make provocations, North Korea will be perceived by people in China as more of a liability than an asset," he was quoted by South Korean news agency Yonhap as saying.
North Korea marked the 101st birthday of founder Kim Il Sung, who died in 1994. The celebrations Monday come amid increased regional tensions. Yonhap reports there are signs North Korea may be preparing for a missile launch, though the South Korean government says there are no signs of imminent war.
North Korea severed most of the official links to South Korea, including ending its commitment to the armistice agreement that halted the Korean War in the 1950s.
North Korea in February carried out its third underground nuclear test.