MOSCOW — In the days after a nuclear accident at a Russian military site on the White Sea, four radiation sensors sending data from Russian territory to an international monitoring network blinked offline.
On Tuesday, a senior Russian official said that the country did not have to share data from those sensors with the network — raising concerns that the Kremlin was withholding information about the severity of radioactive contamination caused by the incident in northern Russia.
The mysterious failure of the four sensors, part of a global system established by a Vienna-based group, had already fueled fears about what took place on Aug. 8. The explosion at a naval weapons range that day killed seven people and released radiation that elevated readings in a city 25 miles away.
Since then, the Russian government has said little, and rumors, incomplete statements and Western analysis have filled the silence. Russian scientists eventually conceded that a nuclear device had released radiation, and American officials, including President Trump, suggested the blast involved a type of novel, nuclear-propelled cruise missile that NATO calls the Skyfall.
President Vladimir V. Putin of Russia said on Monday that the accident posed no risk to the public, although Russian officials have not disclosed how much radiation was released.
Two radiation sensors within the international network and relatively close to the accident, in Dubna and Kirov, went offline on Aug. 10, according to a spokeswoman for the Vienna-based group, the Comprehensive Nuclear Test Ban Treaty Organization. She asked not to be identified in accordance with the treaty organization’s policies.
Another two sensors, in Bilibino and Zalesovo in Siberia, went offline on Aug. 13, according to the treaty monitoring organization. Two sensors resumed functioning on Tuesday, the group said, though it was not immediately clear which ones. The outages were reported Monday by The Wall Street Journal.
After the automated sensors failed, the treaty monitors inquired about the reason. Russian authorities who manage the sensors, which typically send data in real time to Vienna, said they had suffered technical problems — or “communications and network issues” — according to the spokeswoman.
But on Tuesday, a Russian deputy foreign minister, Sergei Ryabkov, appeared to contradict that statement. He told the Interfax news agency that Russia’s transmission of data from the sensors was voluntary, and that Russia had chosen to provide data but was not treaty-bound to do so.
The data might have helped determine the nature of the incident and the extent of radioactive contamination — potentially helping people decide whether to take medicine to protect against radiation, such as iodine, but also possibly revealing military secrets about what happened.
Mr. Ryabkov did not directly address the Vienna-based group’s assertion that Russia had stopped sharing the data after the accident. But his comment suggested it was Moscow’s prerogative, and not a coincidental technical fault at four sensors, that caused the disruption.
“It is necessary to keep in mind that handing over the data from the national segment of the international system of monitoring is an entirely voluntary affair for any country,” Mr. Ryabkov said.
The test ban treaty of 1996 is observed by many states but has not formally taken effect, as some countries with nuclear capabilities, including the United States, have not signed or ratified it. Site inspections would only be possible under the treaty after it takes effect. Russia has signed and ratified the treaty.
Mr. Ryabkov added that the incident near Arkhangelsk “should have no relation at all to the activities” of the test ban monitoring group, as the organization’s mandate is limited to identifying possible nuclear explosions. Russian scientists said on Aug. 11 that the accident occurred as they studied “small-scale sources of energy with the use of fissile materials.”
In his first public comments on the accident, Mr. Putin said on Monday that there was “no threat and no rise in radiation level.” He said that “preventive measures are taken so nothing unexpected happens.” He did not clarify what those measures were.
Newsader Analytics, a Russian news site, posted a grainy video that showed a Russian soldier discussing safety with residents of Nenoksa, a village near the weapons test site. The soldier cautioned them not to handle unknown objects that wash ashore on the beaches of the White Sea, suggesting radiation risks remain.
“This would be unwise, because you would be exposing yourself to the same problems you are afraid of now,” the soldier said about beachcombing. There would be risks, he said, of taking home “objects lying on the beach that at first glance seemed safe.”