China has called on the United States to cancel a planned sale of hundreds of missiles to Taiwan.
US arms sales are also opposed by some in Taiwan
The foreign ministry said the deal would harm peace and stability in the region and damage China-US relations.
The US defence department this week told Congress it planned to sell $421m (£216m) worth of missiles to Taiwan.
China sees Taiwan as part of its territory has threatened to use force if the island ever moved to declare formal independence.
The United States is committed to supplying Taiwan with defensive weapons, and has said it would view any Chinese attack as a matter of "grave concern" to the US.
The Chinese foreign ministry statement said Beijing had lodged a formal complaint with the US over the proposed missile sale.
Ruled by separate governments since end of Chinese civil war in 1949
China considers the island part of its territory
China has offered a "one country, two systems" solution, like Hong Kong
Most people in Taiwan support status quo
"We solemnly demand the leader of the United States... immediately cancel this weapons sale (and) avoid harming the peace and stability of the Taiwan Straits and Sino-US relations," ministry spokesman Qin Gang said.
He said the planned sale would "seriously violate" Washington's previous commitments to reduce arms sales to Taiwan.
The US Department of Defence announced this week that it planned to sell 218 Advanced Medium Range Air-to-Air missiles and 235 Maverick missiles to Taiwan.
The Defence Security Cooperation Agency said the sale would help Taiwan "modernise its armed forces and enhance its defence ability to counter air and ground threats".
"The proposed sale will help improve the security of the recipient and assist in maintaining political stability, military balance, and economic progress in the region," it also said.