A top defense official suggested Russia might have illegally tested low-yield nuclear missiles, effectively violating a 1996 international ban on nuclear tests.

Speaking Wednesday at the Hudson Institute, a Washington think tank, Lt. Gen. Robert P. Ashley Jr., director of the Defense Intelligence Agency (DIA), said the U.S. believes Russia "probably is not adhering to the nuclear testing moratorium." The United Nations Comprehensive Nuclear Test Ban Treaty (CTBT) bans nuclear explosions of any size, for either civilian or military purposes.

"The U.S. has determined that Russia's actions have strained key pillars of arms control architecture," said Ashley. "Russia claims to be developing new warhead designs for strategic systems such as a new high-yield, earth-penetrating warhead to attack hardened military targets like the U.S. allied and Chinese Command and Control facilities."

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Ashley contended that Russia's current testing activities would help the country vastly improve its nuclear weapon capabilities. According to The Wall Street Journal, Trump administration officials say that view is also shared by other intelligence agencies in the U.S.

When pressed on the allegation by Journal reporter Michael Gordon, Ashley would only say Russia had the "capability" to conduct very low-yield nuclear tests, a capability which Russia, China and the United States have long possessed, according to the Arms Control Association. He did not say whether Russia has conducted or is conducting such tests.

"We believe they have the capability in the way they are set up," Ashley said.

For years, Pentagon officials have been expecting the Russians to violate the treaty, according to CBS News' David Martin, but Wednesday's comments mark the first time the U.S. has explicitly said Moscow has failed to observe its commitments under the CTBT. The U.S. has signed but not ratified that treaty.

Critics of the pact, including national security adviser John Bolton, have said the treaty does not properly define a nuclear test and that other countries, including Russia and China, have a vastly different interpretation of what the treaty prohibits. Bolton previously claimed the CTBT offered "illusionary protections."

The Kremlin responded to Wednesday's remarks in kind.

"The statement of the director of the United States Defense Intelligence Agency (DIA) about Russia's alleged secret nuclear tests reveals the U.S. military's declining professionalism," head of the State Duma Defense Committee Vladimir Shamanov told Russia's state-run Interfax news agency.

"He just could not have made a more irresponsible statement. Nuclear tests cannot be carried out secretly with equipment, which is controlling these processes today," Shamanov said Wednesday.

The U.S. has similarly accused Russia of violating the Intermediate-Range Nuclear Forces (INF) agreement. In February, President Trump said Washington was starting the process of withdrawing from the treaty in six months time. Many analysts say abandoning that 1987 treaty could effectively signal the start of a new arms race.

Asked what U.S. policy should be moving forward with regards to the escalating tensions with Russia, foreign policy expert Angela Stent of Georgetown University told CBS News senior national security contributor Michael Morell that the U.S. needs to have "realistic expectations" with Russia on arms control.

"We have a major treaty that's going to expire, the new START Treaty in 2021. These are areas where, as the world's two nuclear superpowers with what, 93% of the weapons, we do need to work with the Russians," Stent said on Morell's podcast "Intelligence Matters."

"We have to define narrowly where we need to work with them, and but not have any illusions about the fact that they're going to move any closer to our view of the world than we have at the present," she said.

Olivia Gazis and Tucker Reals contributed to this report.