The head of the US defence intelligence agency has claimed that Russia has “probably” been violating the nuclear test ban treaty by carrying out low-yield tests in a remote Arctic facility.

The claim by Lt Gen Robert Ashley drew widespread scepticism from nuclear weapons experts who said it appeared to revive earlier allegations made by US hawks that have not been substantiated.

There was no mention of Russian low-yield testing in the annual state department report on arms control compliance, which was issued last month.

Furthermore, the nuclear test watchdog, the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO), said its international monitoring system (IMS) was operating normally and had detected nothing unusual.

“The CTBTO has full confidence in the ability of the IMS to detect nuclear test explosions according to the provisions of the Comprehensive Nuclear-Test-Ban Treaty,” the organisation said in a statement. “The CTBT verification regime is already working and effective, with over 300 monitoring stations deployed around the world and sending data.”

In prepared remarks at the Hudson Institute thinktank in Washington, Ashley said the US expected Russia’s nuclear arsenal to grow “significantly” over the next decade, particularly “non-strategic”, shorter range and lower yield warheads. In order to design new warheads, Ashley said, Russia was breaking the rules of the CTBT, which the country has signed and ratified.

The US has signed but not ratified the treaty, which has yet to enter into force. The nuclear weapons powers have meanwhile been observing a moratorium on testing.

“Russia’s development of new warhead designs and overall stockpile management efforts have been enhanced by its approach to nuclear testing,” Ashley said. “The US believes that Russia probably is not adhering to its nuclear testing moratorium in a manner consistent with the “zero-yield standard”.

Officials have said the allegations refer to a facility in a remote Arctic chain of islands called Novaya Zemlya, which was used by the Soviet Union for nuclear testing.

Asked twice whether the US knew Russia was carrying out low-yield nuclear testing, however, Ashley would not go beyond saying: “They have the capability to do it.”

“This seems to be rumours masquerading as an actual compliance assessment,” said Alexandra Bell, a former state department arms control official and now senior policy director at the Centre for Arms Control and Non-Proliferation. “If you’re truly concerned about this, what concerns me is why wasn’t this in the compliance report that was released less than a month and a half ago. The word testing appears exactly zero times.”

Russia insists it is compliance with the test ban treaty. Pavel Podvig, an independent analyst who runs the Russian Nuclear Forces research project in Geneva, said he was “extremely sceptical” about the US claims.

“It’s not a secret that Russia keeps the Novaya Zemlya site in readiness to resume tests, but that’s what the US official policy is too,” Podvig said. “To make a jump from ‘have the facilities that allow testing’ to ‘probably testing’ is very irresponsible.”