The US has conducted an underground chemical explosion at a test site in Nevada "to improve its ability to detect low-yield nuclear explosions around the world", according to the US Department of Energy's press office.
"These experiments advance our efforts to develop new technology in support of US nuclear non-proliferation goals. They will help reduce global nuclear threats by improving the detection of underground nuclear explosive tests," said Corey Hinderstein, a spokeswoman for the National Nuclear Security Administration (NNSA), a division of the US Department of Energy.
The agency said chemical explosives and radioisotope indicators were used in the experiment.
The tests took place on 18 October, the day the lower house of the Russian parliament passed a bill to withdraw ratification of the Comprehensive Nuclear Test Ban Treaty (CTBT).
The CTBT expands on the 1963 treaty that banned nuclear weapons testing in the atmosphere, in space, and underwater, and includes a ban on underground explosions. Each party to the treaty undertakes not to conduct a nuclear explosion "at any place under its jurisdiction or control". Russia signed the CTBT in 1996 and ratified it in 2000. However, the treaty has never entered into force because India, North Korea and Pakistan have not signed it, and the United States, China, Egypt, Israel and Iran have signed but not ratified it.