The counter on a "peace clock" monument in the city of Hiroshima showing the number of days since the world's last nuclear test has been reset following revelations last week that the U.S. conducted two tests using plutonium last year to examine the capabilities of its nuclear arsenal.

On Tuesday, in the 15th reset since the clock was installed at the Hiroshima Peace Memorial Museum in August 2001, the digital number was changed to 55 from 285, indicating the number of days since Nov. 16, when the United States conducted its latest nuclear weapons capability test.

The clock was last reset on May 24, 2011, following similar U.S. tests with the use of plutonium to examine the effectiveness of its nuclear weapons.

Koichiro Maeda, director of the peace museum, criticized the U.S. for conducting the tests, saying: "I can only say the tests were regrettable. Whatever types of experiments were conducted, I cannot help but feel that the United States intends to keep its nuclear weapons."

The 3.1-meter-tall monument was produced and donated to the museum by a peace group on the 56th anniversary of the Aug. 6, 1945, U.S. atomic bombing of Hiroshima, in the hopes of guarding against fresh nuclear tests.