Russia is likely violating an international treaty banning nuclear tests by conducting secret operations on remote islands above the Arctic Circle, a U.S. intelligence assessment revealed Wednesday.

Analysts who spoke to The Wall Street Journal say they believe Moscow is carrying out very low-yield nuclear tests on Novaya Zemlya, a remote archipelago, in order to bolster their weapons capabilities. Russia, the U.S. and dozens of other countries have previously agreed under the Comprehensive Nuclear Test Ban Treaty to prohibit nuclear tests of any size and related activities that generate explosive yields.

“The United States believes that Russia probably is not adhering to its nuclear testing moratorium in a manner consistent with the ‘zero-yield’ standard,” Lt. Gen. Robert Ashley, the U.S. Defense Intelligence Agency’s director, said in a speech at the Hudson Institute Think Tank.

Trump administration officials told The Wall Street Journal that other intelligence agencies share the same view.

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“Our understanding of nuclear weapon development leads us to believe Russia’s testing activities would help it to improve its nuclear weapons capabilities,” Ashley added.

Those familiar with Russia’s testing would not say exactly how large the operations are. It is also not clear if the U.S. has raised its concerns with the Kremlin.

One Russian Embassy official in Washington denied that his country was violating the terms of the treaty, according to the Journal.

A senior Trump administration official said that the U.S. isn’t interested in resuming nuclear testing, but that it wouldn’t allow Russia and China to get away with any operations that go above the “zero yield” guidelines.

“We do not want to be held to a different standard than Russia and China,” he said.

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The suspicions come months after Secretary of State Mike Pompeo announced that the U.S. is pulling out of a major arms-control treaty with Russia, citing alleged violations.

A U.S.-Russia treaty curtailing the use of long-range nuclear arms is also set to expire in early 2021. Trump administration officials are currently mulling over whether to renew, modify or abandon that agreement.