Russian President Vladimir Putin, right, with Foreign Minister Sergey Lavrov, discusses arms control and other issues with Secretary of State Mike Pompeo on May 14. (Alexander Nemenov/AP)

The United States said Russia is “probably” not adhering to an international treaty banning nuclear weapons testing but stopped short of directly accusing the country of conducting illegal low-yield tests, as a war of allegations over arms control violations escalates between Washington and Moscow.

Lt. Gen. Robert P. Ashley Jr., the director of the Defense Intelligence Agency, said Wednesday in a speech at the Hudson Institute in Washington that Moscow’s questionable approach to nuclear testing has helped Russia enhance new warhead designs. 

Ashley suggested that Russia, in developing new nuclear systems that President Vladimir Putin has touted, is probably not adhering to the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The 1996 pact prohibits “any nuclear weapon test explosion or any other nuclear explosion.” Signatories widely have taken this to mean any test of any size, regardless of yield.

“The United States believes that Russia probably is 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. The United States, by contrast, has forgone such benefits by upholding a zero-yield standard.”

Ashley didn’t say U.S. intelligence agencies possessed specific evidence that Russia had conducted low-yield nuclear tests. But the DIA director said he thinks Russia has “the capability to do that,” suggesting that Russia’s pursuit of a new array of nuclear weapons would probably include such testing. 

Speaking on a panel afterward, Tim Morrison, senior director for weapons of mass destruction and biodefense on the White House National Security Council, added, “We believe Russia has taken actions to improve its nuclear weapons capabilities that run counter or contrary to its own statements regarding the scope of its obligations under the treaty.”

The accusation prompted a rebuke from some Russian officials.

Konstantin Kosachev, head of the foreign relations committee in the Federation Council, Russia’s upper house of parliament, said Russia had obeyed a moratorium on testing since the 1980s.

“Unfounded accusations from the mouths of the Americans have already become an absurd norm in international affairs,” Kosachev said in a post on Facebook.

The Russian Embassy in Washington didn’t immediately respond to a request for comment.

The United States is a signatory of the CTBT, but the Senate has not ratified the treaty. Russia has signed and ratified the pact. The treaty has set the norm for nuclear testing but technically hasn’t entered into force, because eight key nations, including the United States, have yet to ratify it. 

The accusation against Russia comes as Washington and Moscow negotiate the future of the New START agreement, the last big arms-control pact remaining between the two nations, which expires in 2021. The White House hasn’t decided whether to pursue a five-year extension, which would require President Trump and Putin to sign off.

Trump would rather strike a broader agreement that addresses Russia’s large stockpile of smaller “tactical” or “battlefield” weapons, rather than just the strategic systems — intercontinental ballistic missiles, bombers and submarines — included in New START. He also wants to bring China into the fold, possibly conducting a trilateral arms control agreement, a desire he brought up with Putin in a recent phone call.

“His direction is pretty clear: He is interested in looking at the totality of Russian and Chinese programs,” Morrison said, adding that Trump doesn’t think it’s advantageous to continue to “defer the difficult questions.”

Putin has said Russia is prepared to extend New START. But in a 2018 interview with Fox News Channel, he said the United States and Russia must first agree on specifics because Moscow has “some questions” for its American partners. “We think that they are not fully compliant with the treaty, but this is for experts to decide,” he said.

Russian officials have voiced concerns about the way the United States is converting some of its weapons systems from nuclear to conventional variants and verifying those conversions under New START. The United States dismisses Russia’s accusations and says it is fully complying with the accord. 

Morrison said such accusations by Putin and other Russian officials raise questions about whether Moscow in fact wants to extend New START.

“We shouldn’t presuppose that the Russians are interested in extending the treaty,” Morrison said. “If they were, they wouldn’t be creating false narratives about U.S. compliance with the treaty.”

The tension between Washington and Moscow over nuclear weapons comes amid a broader breakdown of the Cold War-era arms control framework that diplomats built painstakingly over decades to reduce the likelihood of a nuclear war.

Citing Russian violations, the Trump administration earlier this year withdrew from the Intermediate-Range Nuclear Forces Treaty, a landmark agreement signed in 1987 by Mikhail Gorbachev and Ronald Reagan. The pact will formally end in August. 

If Trump and Putin decline to extend New START and do not agree on a replacement treaty, the world’s two biggest nuclear powers will lack any legally binding or verifiable limits on their arsenals for the first time since 1972.

The new accusation by the DIA director regarding nuclear testing raised questions for arms control advocates, given that U.S. officials as recently as 2016 have said Russia is complying with the CTBT.

“A key question here is whether there is new evidence that the intelligence community has come up with since 2016, credible evidence, that Russia is conducting nuclear test explosions,” said Daryl G. Kimball, executive director of the Arms Control Association. “Or is this a new interpretation of a known set of facts that has been around for decades?”

For years, a group of American officials and experts has speculated that Russia is conducting very low-yield nuclear tests at its facility at Novaya Zemlya but no evidence has been made public to back those suggestions, said Jeffrey Lewis, a nuclear weapons expert at the Middlebury Institute of International Studies. Lewis said the Russians have acknowledged conducting permitted subcritical experiments there, much like the ones the United States conducts at its U1a Complex in Nevada. 

“Russia’s test site Novaya Zemlya has been active just like the U.S. test site in Nevada,” Lewis said. “There is no evidence that Russia is conducting prohibited activities. But if we really care that much, we could propose reciprocal inspections.”