New START, the last nuclear treaty between Washington and Moscow after decades of agreements dating to the Cold War, is due to expire on Thursday, and with it restrictions on the two top nuclear powers.

"I urge you not to abandon this instrument without seeking to ensure that it is followed up in a concrete and effective manner," the American pope said at his weekly general audience.

"The current situation requires us to do everything possible to avert a new arms race, which further threatens peace between nations," he said.

Leo, the Catholic Church's first American pontiff, said it was "more urgent than ever to replace the logic of fear and mistrust with a shared ethic capable of guiding choices towards the common good".

The Kremlin has offered a one-year extension of the treaty, but while US President Donald Trump said in September that an extension of the New START "sounds like a good idea", little has changed since then.

The treaty, which included a monitoring mechanism, was signed in 2010 by then-Russian President Dmitry Medvedev and his US counterpart Barack Obama.

But Russia suspended monitoring inspections during the Covid-19 pandemic and talks on extending the agreement have broken down in recent years due to tensions over the Ukraine war.

Moscow had also accused Washington of impeding monitoring missions on US soil.

In 2023, Russia froze its participation in New START, but it has continued to voluntarily adhere to the limits set in the treaty.

Moscow has last year tested its latest nuclear weapon carriers without atomic warheads, and Trump said he was moving two nuclear submarines closer to Russia.