TOKYO -- The U.S. has called on Japan to reduce its high levels of stockpiled plutonium, a move that comes as the Trump administration seeks to convince North Korea to abandon its nuclear weapons, Nikkei has learned.

The request was made by the U.S. Department of State and National Security Council ahead of next month's extension of a bilateral nuclear cooperation agreement, according to people familiar with the matter.

Japan has about 47 tons of the radioactive element -- enough to produce around 6,000 nuclear warheads. Foreign and domestic critics have pointed to these reserves as a ready source of bomb-making material should Japan choose to become a nuclear weapons state.

Plutonium production is banned in principle, but energy-poor Japan has been allowed to extract the material from spent nuclear fuel rods under the bilateral pact.

Japan insists that it does not maintain plutonium reserves "without specified purposes." Critics in Japan and elsewhere, including China and some in the U.S. Congress, have expressed concern about the size of these stockpiles.

Now, as U.S. President Donald Trump heads into a meeting with North Korean leader Kim Jong Un that Washington hopes will bring progress toward denuclearization, Japan's exemption form rules aimed at curbing the spread of nuclear weapons material may be called into question.

The U.S. has asked Japan to impose a cap on its plutonium stockpiles. The Trump administration also wants to issue a joint statement, stating that Japan's plutonium supply is for peaceful use, when the nuclear cooperation pact is renewed, according to people familiar with the matter.

In response, Japan's nuclear regulator is expected to adopt as early as this month a policy of reducing the stockpiles and not allowing them to exceed current levels. The start of operations at the Rokkasho reprocessing facility in northern Japan, which was intended to mass produce plutonium from spent nuclear fuel, is also likely to be delayed. These moves would amount to a de facto cap on the country's plutonium stockpiles.

Tokyo "will respond in good faith to the [U.S.] request, but this will also require efforts by power companies," said a Japanese government source. "This isn't something that is going to happen overnight."

The Japanese government has asked the Federation of Electric Power Companies, whose members include the country's nuclear plant operators, to reduce plutonium reserves. Two utilities in western Japan have been asked to consider using so-called mixed oxide fuel, a blend of uranium and plutonium, in reactors coming back online.

But utilities have in principle used MOX fuel made with plutonium from the own plants. Moreover, with most of the country's reactors idle amid concerns over safety since the 2011 Fukushima disaster, the stockpiles look unlikely to fall soon.

Some in the prime minister's office have expressed concern that the plutonium issue could doom Japan-U.S. nuclear agreement. The pact will renew automatically on July 16, but can be halted if if either side gives notice within six months.

"If the U.S. calls off the deal, the foundations of Japan's fuel cycle will crumble," said a government source.