Japan strictly controls the export of arms under its pacifist constitution, which limits its military capacity to ostensibly defensive measures.

The country's defence industry is small, with the only customer being the Japanese military and the market estimated at around 3 trillion yen ($20 billion) annually -- less than some individual US defence contractors' yearly revenues.

Tokyo moved on Friday "to add a new rule" to existing regulations on defence equipment exports, according to a national security official in the prime minister's cabinet who spoke on the condition of anonymity.

With the new rule, set to be approved at a cabinet meeting later Friday, Japan "will be able to export arms which were domestically produced under licence of a foreign company to the licensing country", the official told AFP.

Japan produces the Patriot Advanced Capability-3 (PAC3) surface-to-air missile defence system, paying a licence fee to US defence firm Lockheed Martin which developed the system.

"Theoretically, the new rule will enable export" of the PAC3 to the United States, the official said.

Japan's plan to export the PAC3 system to the United States would be its first export of lethal arms since the end of World War II, local media have reported.

A senior ruling party official told reporters this week that the export plan was at the request of Washington, Kyodo News reported.

US President Joe Biden raised the issue with Prime Minister Fumio Kishida at a meeting at Camp David in August, as well as during an economic summit in San Francisco last month, The Washington Post reported this week, citing unnamed US officials.

Washington is increasingly looking to its allies to supply sophisticated weapons against the backdrop of a shortfall in Ukraine's air defences, and South Korea has quietly pledged to provide hundreds of thousands of rounds of artillery ammunition to Kyiv over the past year, the newspaper said.

Japan used to ban all exports of defence equipment but in 2014 the late prime minister Shinzo Abe's cabinet loosened the rules.