Vice President Dick Cheney has told Chinese officials it is important to move more aggressively in efforts to get North Korea to dismantle its nuclear weapons programs.

Mr. Cheney voiced concerns to Chinese leaders, amid what U.S. officials say is growing evidence that North Korea has probably already developed nuclear weapons.

Speaking after Mr. Cheney's meeting with Chinese President Hu Jintao and Prime Minister Wen Jiabao, a U.S. official said the United States thinks it is important to move aggressively because "time is not necessarily on our side."

China has been the crucial link in bringing the North Koreans to negotiations also involving Japan, South Korea, Russia and the United States.

As North Korea's closest ally and its main supplier of food and fuel, Beijing is believed to be in the best position to influence Pyongyang. However, some analysts say China's ability to persuade the largely reclusive nation is limited.

Jin Canrong is a professor of international studies at Remin University in Beijing. He says China cannot do much more than urge North Korea to resume negotiations, and encourage it to take a more constructive attitude. He says China has opted for a cautious approach.

"China does not want to be the main target of North Korea. ? Historically speaking, China has no immediate influence within that country," said Professor Jin. "North Korea is a very independent country and very sensitive to pressure from the outside world. So, if we push them too hard, it is very possible we will not achieve our goal."

Negotiations have centered on demands by the United States for North Korea to completely and verifiably dismantle its nuclear weapons programs. North Korea admits it has a nuclear program, and says it is building a deterrent force to prevent what its leaders say is a possible U.S. attack.

Diplomats from some of the nations involved are considering holding low-level working group meetings in Beijing as early as the end of this month.

Mr. Cheney left Beijing Tuesday for the eastern city of Shanghai, his latest stop on an Asian tour that began in Tokyo. After Shanghai, Mr. Cheney heads to South Korea.