Washington(CNN) The arms control framework curbing a US-Russian arms race came under further strain this week as Russian President Vladimir Putin took formal steps to withdraw from a landmark nuclear missile treaty and the US charged that Moscow is violating a separate pact that bans all nuclear explosions.

Putin submitted a bill to parliament Thursday to withdraw Russia from the Intermediate-Range Nuclear Forces Treaty, following the Trump administration's February announcement that it would pull out of the pact because Russia has violated it since 2014, which Moscow denies.

On Wednesday, the US military's top intelligence officer said the Trump administration believes Moscow is violating a treaty banning all nuclear explosions, a claim that arms control experts questioned and the international body that monitors the treaty could not confirm.

Lt. Gen. Robert Ashley, director of the US Defense Intelligence Agency, told an audience at the Hudson Institute that the US believes Moscow is violating the Comprehensive Nuclear Test Ban Treaty's "zero yield" standard, which forbids tests strong enough to create a self-sustaining chain reaction that can lead to a nuclear explosion.

No evidence

He argued that the tests were helping Moscow develop new nuclear weapons capabilities.

"The United States believes that Russia is probably not adhering to the nuclear testing moratorium in a manner consistent with the zero-yield standard," Ashley said. "Our understanding of nuclear weapon development leads us to believe Russia's testing activities would help it improve its nuclear weapon capabilities."

Ashley presented no evidence for the statement and when pressed, wouldn't confirm that Russia is conducting these tests, saying only that Moscow could conduct them.

"It is our belief that they are set up in such a way that they are able to operate beyond a way that would be necessary for a zero-yield," Ashley said, "and so the facilities they are operating have that capacity to operate in something other than zero-yield."

Ashley's claim raised eyebrows among arms control experts, who said the general offered nothing to back up his assertion that Russia is breaching the 1996 treaty and that the development of new nuclear capabilities would require tests too big for Moscow to hide.

These experts questioned the administration's intent, pointing both to national security adviser John Bolton's well-known dislike of arms control treaties, as well as the Trump administration's hostility to all manner of international agreements.

And they flagged a US report released just a month earlier on compliance with arms control, non-proliferation and disarmament treaties that didn't mention the Comprehensive Nuclear Test Ban Treaty or Russian testing that violates it.

"This accusation comes with absolutely zero evidence to support it," said Alexandra Bell, senior policy director at the Center for Arms Control and Non-Proliferation. She noted the lack of details Ashley offered. "There was no mention of time or a specific incident or any clear indication that we've consulted with our allies."

"The word 'testing' didn't even make it into the report" on compliance released last month, Bell noted. This year's version of the report was unusually short by previous years' standards and was criticized for potentially being politicized in its approach to Iran.

Bell said of Ashley's charges against Russia that "if we're ready to talk about it publicly, why didn't it make it into that compliance report?"

No unusual events

Daryl Kimball, executive director of the Arms Control Association, said that "Ashley and the administration appear to be stretching the facts beyond what the evidence they presented supports. It is absolutely no surprise that Russia and the US and China have the capability to conduct a nuclear test explosion. The question is: is Russia actually doing this?"

The Comprehensive Test Ban Treaty Organization, the independent body which watches for violations with over 300 monitoring stations around the world, said it had seen no unusual activity.

"The CTBTO's International Monitoring System (IMS) is operating as normal and has not detected any unusual event," the Vienna-based group said in a statement. "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 statement added that the "verification regime is already working and effective."

The Russian Embassy did not respond to requests for comment.

Stephen Young of the Union of Concerned Scientists, said "the assertion that Russia is undertaking activities that are violating the treaty can't be dismissed out of hand," but he added that he was concerned "this is the first step to get the US to withdraw from the treaty by accusing Russia of violating it."

Nuclear modernization

Like other arms control analysts, he pointed to Bolton. "This administration, in particular John Bolton, does not like treaties of any kind," Young said, "so there is concern that this rather sudden announcement about Russian activities is merely a subtext of a goal of John Bolton's of getting the US out" of the treaty, which it has signed, but not ratified.

Ashley's claim comes as longstanding arms control agreements between US and Russia are under strain or unraveling and both sides are focused on modernizing their arsenals.

The US announced in February that it will leave the 1987 INF Treaty, a centerpiece of European security since the Cold War that bans ground-launched missiles with a range of between roughly 300 to 3,400 miles and 500 and 5,500 kilometers. The bill Putin submitted Thursday comes into force as soon as it passes the upper house of parliament and is officially published in a state newspaper.

Washington has also indicated that it wants Moscow to renegotiate the landmark New START Treaty, which focused on reducing strategic nuclear stockpiles and expires in 2021, and include China -- a scenario that's unlikely to happen, experts say.

In the meantime, the US is one of eight nations known as the "hold-out states" that have to ratify the Comprehensive Nuclear Test Ban Treaty before it can enter into force. The others are China, Egypt, India, Iran, Israel, North Korea, and Pakistan. Russia has both signed and ratified the agreement. Ratifying the treaty would allow Washington to demand the kind of short-notice inspections that could prove whether Russia was complying or not.

'Tests they could not hide'

The CTBTO noted in its statement that "the Treaty can only be fully implemented after its entry into force, when the prohibition becomes legally binding for all States Parties. This is also true of the final verification measure of the Treaty, the provisions for on-site inspections, which would allow for on-site visits at short notice if requested by any State Party."

Asked about Ashley's comments Wednesday, State Department spokesperson Morgan Ortagus said the agency was "certainly alarmed that they (Russia) continue to disregard their international obligations as it relates to arms control."

Ortagus referred to the Defense Intelligence Agency on Ashley's specific comments, but said that the State Department has repeatedly noted that Russia "routinely disregards its international security and arms obligations."

Even so, Young of the Union of Concerned Scientists said that "if Russia were seeking to develop any significant new nuclear capabilities, that would require tests that they could not hide. And Russia can't do tests this small that would lead to any significant change in the overall nuclear deterrent equation or negatively impact US security."

Bell agreed, saying that "even if Russia was messing around in this range [of tests] it wouldn't be militarily significant."