The U.S. government notified Congress Wednesday of the planned sale of advanced torpedoes worth $180 million, prompting protests from Beijing—which does not recognize Taiwan as an independent nation.

Chinese Foreign Ministry spokesperson Zhao Lijian told reporters at a briefing Thursday that Beijing had lodged “solemn representations” with the U.S. to complain about the planned sale.

Zhao said Beijing firmly opposes arms sales to Taiwan, and warned that the deal could undermine both U.S.-China relations and regional stability.

The U.S. Defense Security Cooperation Agency released a statement Wednesday noting it had “delivered the required certification notifying Congress of this possible sale” of 18 MK-48 Mod6 Advanced Technology Heavy Weight Torpedoes and related equipment, worth some $180 million.

The agency said the deal is in support of Taiwan’s “continuing efforts to modernize its armed forces and to maintain a credible defensive capability.”

The U.S. has no official diplomatic ties with Taiwan, but is required by law to provide the island nation with the military means to defend itself.

China considers the democratic nation to be a wayward province, and under its “One China” policy intends to bring the country back under Beijing’s control.

Taiwan—officially called the Republic of China (RoC)—has been independent for more than 70 years, having emerged from the last bastion of the nationalist forces that lost the Chinese Civil War to the Communist Party. It became the RoC capital in 1949.

Beijing has consistently vowed to regain control of the island, either by diplomatic or military means. Taiwan sits 80 miles from the Chinese coast across the Taiwan Strait, a strategic waterway where Chinese, U.S. and Taiwanese forces regularly conduct military operations.

The notice of the torpedo deal was issued on the same day that Taiwanese President Tsai Ing-wen was sworn in for her second term, having triumphed in January’s election. Tsai—the leader of the nationalist and liberal Democratic Progressive Party—said she strongly rejected Beijing’s claim of sovereignty over the island.

Secretary of State Mike Pompeo congratulated Tsai on her second term, noting: “The United States has long considered Taiwan a force for good in the world and a reliable partner. Support for Taiwan in the United States is bipartisan and unanimous.”

Beijing reacted angrily, expressing “strong indignation and condemnation.” Zhao told reporters Wednesday that China “will take necessary countermeasures, and the consequences will be borne by the U.S. side.”