Top officials of Hiroshima and Nagasaki, two Japanese cities that suffered atomic bombing in World War II, on Tuesday protested against the U.S.'s conducting of two rounds of tests on the properties of plutonium last year, Kyodo News reported.

Hiroshima Mayor Kazumi Matsui, Hiroshima Governor Hidehiko Yuzaki and Nagasaki Mayor Tomihisa Taue lodged written protests in a letter to the U.S. President Barack Obama a day after the National Nuclear Safety Administration announced that a U.S. lab conducted tests on the so-called Z machine to test the properties of plutonium with X-ray emissions.

In his letter, Taue urged Obama to halt all types of experiments relating to nuclear arms and play a leading role in wiping out nuclear weapons.

The U.S. Sandia National Laboratories in New Mexico conducted tests using plutonium to examine the capabilities of nuclear weapons twice in the October-December period last year. The United States has reportedly continued such plutonium tests to see if its nuclear weapons remain in an operational state.