Mayors of Hiroshima and Nagasaki, two Japanese cities that suffered atomic bombing in World War II, on Wednesday sent protest letters to U.S. President Barack Obama after the United States revealed it had conducted a plutonium test between July and September, Kyodo News reported.

Hiroshima Mayor Kazumi Matsui said in his letter that the test showed the "United States intends to cling to its nuclear stockpile indefinitely, and such actions are completely unacceptable."

The test has betrayed the hopes of the survivors of the bombing and millions of others who seek the elimination of nuclear weapons, Matsui said, adding that it also arouse suspicion regarding Obama' s commitment to a world without nuclear weapons.

Nagasaki Mayor Tomihisa Taue also sent a letter to Obama with a similar message, the report said.

The U.S. National Nuclear Security Administration has said on its website that a plutonium test involving a device called a "Z machine" was conducted in the July-September period, following the one in the April-June quarter.