But with a key nuclear deal set to expire next year, the study warns of a potential new arms race.
“The loss of key channels of communication between Russia and the USA … could potentially lead to a new nuclear arms race,” said Shannon Kile, co-author of the report.
Meanwhile, nuclear powers China and India are increasing and modernising their arsenals.
“China is in the middle of a significant modernisation of its nuclear arsenal. It is developing a so-called nuclear triad for the first time, made up of new land- and sea-based missiles and nuclear-capable aircraft,” the report said.
The country has rejected calls by the US that it joins nuclear arms reduction talks.
The number of nuclear warheads throughout the world declined in the past 12 months.
At the beginning of 2020, the US, Russia, Britain, China, India, Pakistan, Israel and North Korea together had 13,400 nuclear arms,, 465 fewer than at the start of 2019.
The decline was largely due to reductions by the US and Russia.
But the reports says both nations have “extensive and expensive programs” underway to modernise their nuclear weapons.