The United States has approved a possible $100 million sale of equipment and services to Taiwan to "sustain, maintain, and improve" the Patriot missile defence system used by the self-ruled island claimed by China, the Pentagon said on Monday.

A statement from the US Defense Security Cooperation Agency said it had delivered the required certification notifying Congress following State Department approval for the sale, which was requested by Taiwan's de facto embassy in Washington.

Upgrades to the Patriot Air Defense System would "help improve the security of the recipient and assist in maintaining political stability, military balance, economic and progress in the region," DSCA said in a statement.

"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 agency said.

The main contractors would be Raytheon Technologies and Lockheed Martin, it said.

Taiwan's Defense Ministry has said the decision to obtain newer Patriot missiles was made during a 2019 meeting with US officials in the administration of President Donald Trump.

The democratically governed island has complained of repeated missions by China's air force in its air defence zone, part of what Washington sees as Beijing's effort to pressure Taipei into accepting its sovereignty.

The United States, like most countries, does not have official relations with Taiwan, but Washington is its biggest backer and is bound by law to provide it with means to defend itself.

US officials have been pushing Taiwan to modernise its military so it can become a "porcupine", hard for China to attack, and such arms sales always anger China.

China's ambassador to the United States said last month that the two super powers could end up in a military conflict if Washington encourages Taiwan's independence.