China's leaders have put pressure on US Vice-President Dick Cheney over US policy on Taiwan.
Taiwan dominated Mr Cheney's meeting with Hu Jintao
President Hu Jintao urged Mr Cheney to "oppose Taiwan independence" and "avoid sending the wrong signals" to Taiwan's authorities by selling them arms.
Mr Cheney said the US did not back independence for the island but opposed the use of force in any reunification.
Taiwan, which China regards as a renegade province, has long been a sticking point in US-Sino relations.
Mr Cheney also raised the issue of the North Korean nuclear crisis, saying it was important to move more aggressively to resolve it.
"Time is not necessarily on our side," a senior administration official said after the talks.
China has proved a crucial player in the dialogue with North Korea, having hosted two rounds of six-party talks on the nuclear issue in Beijing.
Mr Cheney met China's three most powerful men - President Hu Jintao, Premier Wen Jiabao and military chief Jiang Zemin - in the space of just a few hours on Wednesday morning.
According to our correspondent in Beijing, Taiwan was the issue which dominated the conversation.
Mr Cheney said that disagreements over the island had
been "an issue between the US and China going back 50 years".
He said he did not come to Beijing "expecting to alter
Chinese policy. I came with a mission to make clear what our views
were. I think we achieved that."
Mr Cheney is due in Shanghai later on Wednesday
President Hu told Mr Cheney: "We hope that the United States can observe its commitment to adhere to the one-China policy." He was referring to China's insistence that Taiwan and the mainland are one country.
Earlier, Chinese Vice-President Zeng Qinghong urged the US to stop selling arms to Taiwan. US law currently requires Washington to provide Taiwan with the weapons it needs to defend itself.
Mr Cheney is said to have offered reassurance that Washington did not support any moves towards Taiwanese independence, nor any action which changed the relationship between the island and the mainland.
Beijing also wants the US to put more pressure on Taiwan's President, Chen Shui-bian, urging him not to move towards independence.
On Wednesday, China's Taiwan Affairs office criticised Mr Chen, accusing him of undermining peace and stability between China and Taiwan.
A spokesman for the Taiwan Affairs office, Li Weiyi, told reporters that Mr Chen's plans for Taiwan's future were designed to hoodwink public opinion.
Mr Li said no one should underestimate the determination of China to safeguard its sovereignty at any cost.
Another topic on the agenda during Mr Cheney's visit was trade.
Washington is concerned about its big trade deficit with China. The US has urged Beijing to buy more American goods like aeroplanes and power plants.
Mr Cheney also urged China to let financial markets determine the value of the yuan.
Washington wants Beijing to end the yuan's peg to the dollar to make it more flexible amid criticism from some US industries that the yuan is keeping Chinese exports unfairly cheap.
In return, Wen Jiabao appealed for Washington to lift restrictions on high-technology exports to China.
Current rules bar China from purchasing US-made
supercomputers and other technology that might improve its
Mr Cheney is in China for three days, on the second leg of a week-long tour of Asia. He is due to travel to Shanghai later on Wednesday.