Meantime, Leaked U.S. diplomatic documents revealed signs of a rift between China and North Korea, which is a striking contrast from officials' statements stressing their strong, historical ties. In the leaked documents, a Chinese envoy mocks North Korea as a "spoiled child."
The whistle-blowing WikiLeaks website showed a frustrated China, as the country mulls over the idea of accepting a future Korean peninsula unified under South Korean rule.
Next week, the U.S., South Korea and Japan have agreed to meet in Washington.
Topics will include the North's recent deadly attack on a South Korean island and its nuclear weapons program.
Also, the U.S. and South Korea are continuing to hold military drills which the North has warned could spark a war.
The Associated Press Contributed to this report.