The United States approved the sale of 18 heavyweight torpedoes to Taiwan for $180 million, the State Department said Wednesday, in a move that could further strain Washington's already tense relationship with Beijing.

The US framed the sale as a mutually beneficial transaction.

"This proposed sale serves US national, economic, and security interests by supporting the recipient's continuing efforts to modernize its armed forces and to maintain a credible defensive capability," the State Department said.

The sale "will help improve the security of the recipient and assist in maintaining political stability, military balance, and economic progress in the region," the statement added, a reference to Washington's strategic priority of countering Beijing's influence.

The MK-48 torpedoes - which can be launched from a submarine - are being provided from existing US Navy stock, the State Department said.