"Russia declared a moratorium on nuclear weapon testing in 1991...However, during the 1995-2018 timeframe, Russia probably conducted nuclear weapons-related tests at the Novaya Zemlya Nuclear Test Site," the report said. "Depending on the nature of these tests, they could raise concerns regarding Russia’s compliance with its TTBT notification obligations."
The report said more information on this situation is classified.
The TTBT prohibits nuclear weapons tests of having a yield exceeding 150 kilotons, the State Department said on its website. It points out that both parties agreed that one or two slight, unintended breaches of the 150 kiloton threshold per year would not be considered a violation of the treaty. They also agreed that any cause of concern would be subject for consultations at the request of either party.
Under the treaty, the United States and Russia agreed to exchange data on a certain number of nuclear weapons tests for calibration purposes to monitor each other's yields of explosions at the specified test sites and the seismic signals emitted.