The mayors of the only two cities to experience atomic bombings sent protest letters to U.S. President Barack Obama on Wednesday after the United States revealed it conducted an experiment using a small amount of plutonium to examine the capabilities of its nuclear weapons between July and September.

In his letter, Hiroshima Mayor Kazumi Matsui denounced the experiment as showing the "United States intends to cling to its nuclear stockpile indefinitely, and such actions are completely unacceptable."

"They have betrayed the hopes of the survivors of the bombing and the millions of others who seek the elimination of nuclear weapons, and also arouse suspicion regarding your commitment to a world without nuclear weapons, which you expressed in your 2009 Prague speech," Matsui said.

Nagasaki Mayor Tomihisa Taue and Hiroyuki Itasaka, chairman of the Nagasaki Municipal Assembly, sent a joint letter to Obama containing a similar message.

Hiroshima was hit by an atomic bomb on Aug. 6, 1945, and Nagasaki was hit three days later.

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 quarter, following one the previous quarter.

Z machines generate X-rays to start fusion reactions so the performance of nuclear weapons can be assessed.