WASHINGTON — The Trump administration believes Russia has restarted very low-yield nuclear tests, officials said on Wednesday in a finding that could be used to renew in earnest the arms race between Moscow and Washington.

But the significance of the statements by the director of the Defense Intelligence Agency and a senior National Security Council official was immediately debated by nuclear weapons experts.

Some experts said claims of low-yield tests would be nothing new. Intelligence officials and nuclear analysts in Washington have long raised the possibility of such violations going back nearly two decades, to when Russia ratified the Comprehensive Nuclear Test Ban Treaty in 2000.

Other nuclear weapons experts have argued that significant Russian cheating on the treaty is unlikely because the designs of the country’s nuclear warheads tend to be very robust. The small returns, they have said, would make the geopolitical costs of getting caught prohibitively high.