US Secretary of State Condoleezza Rice has urged Chinese leaders to put pressure on North Korea to move ahead quickly with nuclear disarmament.
Ms Rice met President Hu Jintao during her visit
She was speaking during a two-day visit to Beijing, her first for over a year.
Ms Rice met President Hu Jintao and Premier Wen Jiabao, and held talks with foreign minister Yang Jiechi.
She also used the visit to raise the issue of human rights and criticise Taiwan's plans to hold a referendum on UN membership.
Ms Rice told reporters that she had held "very good discussions about a number of issues" with her Chinese counterparts.
"I'm quite certain that when China and the United States co-operate, we are better able to resolve some of the really complex and difficult issues that face the international system," she said.
China is North Korea's closest ally and chairs the six-nation talks which produced a historic disarmament agreement a year ago.
Under the agreement, Pyongyang was promised aid and diplomatic concessions in exchange for disabling its nuclear plant at Yongbyon and submitting a full declaration of all its nuclear activities by the end of the year.
But when the end of 2007 came, the disabling process had not been completed, nor was there a full declaration to the Americans' satisfaction.
To make matters worse, the process of disabling Yongbyon has slowed since the beginning of the year, as Pyongyang claims the promised aid has not been arriving fast enough.
N Korea to "shut down and seal" Yongbyon reactor, then disable all nuclear facilities
In return, it will be given 1m tons of heavy fuel oil
Under an earlier 2005 deal, N Korea agreed to end nuclear programme and return to non-proliferation treaty
N Korea's demand for a light water reactor to be discussed at an "appropriate time"
"I'm expecting from China what I'm expecting from others - that we will use all influence possible with the North Koreans to convince them that it's time to move forward," Ms Rice said at a joint press conference with Chinese Foreign Minister Yang Jiechi.
Mr Jang said Beijing had been dealing closely with Pyongyang in regard to its disarmament process, and would "continue to play and important role in moving this forward".
He added that he hoped the declaration could be "implemented in a balanced, integrated way".
Mr Rice also used her visit to raise US concerns about human rights and religious freedoms in China, stressing the "importance of resuming a human rights dialogue" between the two countries.
"We do this in a spirit of respect but these issues are very near and dear to America," she said.
Ms Rice's delegation is reported to have drawn attention to several specific cases of political activists detained by China. The Secretary of State also criticised a plan by Taiwan to stage a referendum on membership of the UN. The plan has angered Beijing which considers the island to be a part of China.
Ms Rice said human rights were "very dear" to America
"Taiwan is a democratic entity that will have to make its own decision, but we believe this referendum is not going to help anyone. In fact, it should not be held," said Ms Rice.
Ms Rice flew into China from South Korea, where she attended the inauguration ceremony of President Lee Myung-bak on Monday.
She will travel to Japan on Wednesday.