Japan said on Friday it would prepare to ship Patriot air defence missiles to the United States after revising its arms export guidelines earlier that day, in the pacifist nation's first major overhaul of such export curbs in nine years.

The announcement comes as Japan, which had long adopted a stance of not allowing exports of deadly weapons, seeks to boost its defence industry amid an increasingly tense security environment in the Indo-Pacific.

"This holds significant meaning in terms of further strengthening the Japan-U.S. alliance. It will contribute not only to Japan's security but also the peace and stability of the wider Indo-Pacific region," Chief Cabinet Secretary Yoshimasa Hayashi said following the announcement.

Although Japan's new export controls still prevent it from shipping weapons to countries that are at war, it may indirectly benefit Ukraine in its war with Russia as it gives the United States extra capacity to provide military aid to Kyiv.

The United States welcomed the move, saying it would replenish U.S. inventories.

"This decision will contribute to the security of Japan and to peace and stability in the Indo-Pacific region by ensuring that U.S. forces, in close cooperation with the Japan Self-Defense Forces, will continue to maintain a credible deterrence and response capability," White House national security adviser Jake Sullivan said in a statement.

A Japanese government official, who declined to be named because of the sensitivity of the matter, said the missiles would be used to re-stock the U.S. military's arsenal for the safety and security of the Indo-Pacific.

The Patriot system is one of an array of sophisticated air defence units supplied by the West to help Ukraine after Russia attacked.

Under previous rules, Japan could only export components of arms and was prohibited from delivering completed products.

The revised guidelines now allow Tokyo to export completed products to countries where patent holders are based. Any re-export to third countries would require permission from Tokyo.

Japan produces Patriot missiles under licence from U.S. firms Raytheon and Lockheed Martin.

Tokyo is also considering exporting 155mm artillery shells, manufactured under BAE Systems' licence, to the United Kingdom, the Financial Times reported on Thursday.

The Japanese government official said exports are considered once the country where the license holder is based makes a formal request, adding that the United States has so far been the only country to make such a request.

U.S. Ambassador to Japan Rahm Emanuel, in a post on X, called it a "historic decision" and a "significant example of Japan's shared commitment to deterrence," adding: "Japan is undertaking a once-in-a-generation defense modernization."

Japan's ruling party has been mulling various changes to export controls for several months, as the rules potentially stand in the way of the ability to export next-generation fighter jets in development with Britain and Italy.