Russia has secretly conducted low-yield nuclear tests of some undetermined magnitude in an effort to upgrade its nuclear arsenal, according to a new U.S. intelligence assessment. The Wall Street Journal reported Wednesday that the supposed testing would be the first time Moscow has failed in its compliance with the Comprehensive Nuclear Test Ban Treaty and challenges the Putin government’s claims to have strictly adhered to the arms-control accord. “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 director of the U.S. Defense Intelligence Agency, was prepared to say, according to remarks in a speech set for Wednesday. Administration officials would not specify the size of the Russian tests, nor would they say if the U.S. has raised its concerns directly with Moscow.

The alleged tests come after the Trump administration said it would withdraw from a treaty on intermediate-range nuclear forces because Russia had allegedly violated the agreement. A senior Trump administration official told the Journal that the U.S. isn’t interested in resuming nuclear testing, but allowing tests by adversaries, such as Russia and China, above the “zero yield” would be indefensible. “We do not want to be held to a different standard than Russia and China,” he said.