US President Donald Trump has instructed the country`s Department of War to immediately resume testing nuclear weapons in a decision that has alarmed disarmament advocates and global security experts.

Posting on his Truth Social platform on Thursday, Trump said he had issued the order "because of other countries testing programs."

"That process will begin immediately," he added.

The US president described Russia and China as respectively the second and third biggest nuclear armed powers in the world, alleging that if Washington did not resume the testing, the countries would catch up with it "within five years."

The testing process is expected to provide data on how new warheads function and whether aging stockpiles remained reliable.

Trump`s remarks marked the most direct US call for renewed nuclear testing since Washington conducted its last live detonation in 1992.

Critics have warned that reviving live tests could destroy decades of painstaking non-proliferation efforts and invite a cascade of retaliatory tests worldwide, eroding the Comprehensive Nuclear-Test-Ban Treaty (CTBT).

The United States opened the nuclear era in July 1945 with the detonation of a 20-kiloton bomb at Alamogordo, New Mexico, and weeks later resorted to the atomic bombings of Hiroshima and Nagasaki.

The latter catastrophe has etched Washington`s name in history as the only party in the world to ever actually deploy the non-conventional arms.

Observers say Trump`s Thursday move threatens to undo efforts by generations of global leaders to ensure the tragedy would never be repeated.

They have also warned about potential efforts by the Israeli regime, the US`s closest ally in West Asia and the only possessor of nuclear arms in the region, to try to justify further enhancing its deadly nuclear arsenal using the knowhow, which is to be acquired by Washington from the tests.

Trump, however, alleged that he "HATED" to issue the order "because of the tremendous destructive power," but "had no choice!" because of his self-proclaimed fear of other nuclear armed powers` catching up with Washington.

Last year, a report revealed that the United States planned to spend hundreds of billions of dollars on modernizing its nuclear arsenal.