Without the New START treaty, which caps the number of deployed nuclear warheads at 1,550 on each side, there will be no limits on the U.S. and Russian arsenals.

The United States accused Beijing on Friday of conducting a secret nuclear test in 2020 as it called for a new, broader arms control treaty that would bring in China as well as Russia.

On Wednesday, Pope Leo urged the US and Russia to renew the treaty, saying the current world situation required "calls for doing everything possible to avert a new arms race".

Russia has repeatedly struck Ukraine's energy infrastructure, risking a nuclear disaster.