China's President Hu Jintao has voiced serious concern over US support for Taiwan, in talks with National Security Adviser Condoleezza Rice.
Rice stressed ties with China remain important
Washington provides Taiwan with advanced weapons to protect it from any attack from Beijing, which regards Taiwan as a renegade province.
Ms Rice said she thought the US-China relationship was developing in a "very promising way".
She is on a three-nation tour of Asia, and is due in Seoul later on Friday.
A BBC correspondent in Beijing, Rupert Wingfield-Hayes, says Miss Rice may have believed she was in Beijing to discuss North Korea's nuclear weapons programme, but her Chinese hosts were having none of it.
The US is on the verge of a deal to sell billions of dollars of new weaponry to Taiwan, and Beijing is determined to do all it can to stop it, our correspondent says.
Mr Hu told Ms Rice that her visit was "helpful to fully understand(ing) China's aspiration in promoting Sino-US ties and the country's 'serious concern' over the Taiwan issue," according to Chinese state news agency Xinhua.
The evening news on China's state TV reflected an apparently stronger line, quoting Chinese military chief Jiang Zemin, who Ms Rice met on Thursday, as saying: "Chinese people are seriously concerned over and dissatisfied about US sales of advanced weapons to Taiwan."
Mr Jiang reportedly told Ms Rice that China would "not sit back and watch and do nothing if the Taiwan authorities cling obstinately to their push for Taiwan independence and if foreign forces meddle and support" the island.
Washington has no formal relations with Taiwan - choosing ties with Beijing instead - but the US is its main arms supplier, intent on maintaining security in the region.
Ms Rice said Washington's relationship with China remained important to the Bush administration.
"The president wants to make very certain that we have continuous discussions about our relationship and that we do them in an atmosphere of mutual respect," she said.
Ms Rice also reportedly discussed North Korea's nuclear ambitions with Mr Jiang.
Beijing has played a key role as facilitator to six-nation talks between Pyongyang and the US and its allies in the region.