A US report says the country held a subcritical nuclear test in the state of Nevada last December.
The test was the first of its kind in 5 years, the 28th by the United States, and the first under the administration of President Donald Trump.
According to the report by the Department of Energy's National Nuclear Security Administration, the experiment called Vega was held at a nuclear test site in Nevada.
Vega involved new explosives used to create powerful impacts on plutonium, and an examination of a plutonium implosion. The NNSA called the test an important step toward enhancing the performance of nuclear weapons.
The US government in February announced a new nuclear strategy of enhancing deterrence capabilities to counter Russia and China, by such means as increasing low-yield nuclear arsenals.
The move marks a shift from the policy of former president Barack Obama, who advocated a world without nuclear weapons.
Observers say antinuclear groups are expected to criticize the Trump administration for boosting nuclear weapon capabilities while pressing for denuclearization of North Korea.