VIENNA — Radioactive sensors are transmitting again at least one Russian monitoring station after four of them went offline following a mysterious blast in the country's far north, its network operator said Tuesday.

The Comprehensive Nuclear Test-Ban Treaty Organization, or CTBTO, operates the stations to check that countries aren't testing nuclear weapons as part of a 1990s pact.

Four of these stations based in Russia went silent in the days after the Aug. 8 explosion, which occurred during a rocket engine test. That silence fueled suspicions among nuclear analysts that Russia tampered with the stations, since it operates them.

The blast killed five employees of Russia's nuclear agency Rosatom but Moscow has not explained why the incident caused a spike in radiation levels in a nearby city.

U.S.-based nuclear experts suspect Russia was testing a nuclear-powered cruise missile.

CTBTO chief Lassina Zerbo said in a tweet that two stations, Peleduy and Bilibino, "have resumed operations in #Russia & are currently backfilling data." He added that there had been "excellent cooperation & support from our Russian station operators."

RN stations RUP 56 (Peleduy) and RUP 57 (Bilibino) have resumed operations in #Russia🇷🇺 & are currently backfilling data to the PTS. Excellent cooperation & support from our Russian station operators under «The Provisional Operation Status of the #IIMS» — Lassina Zerbo (@SinaZerbo) August 20, 2019

Bilibino is one of the four stations that the Vienna-based CTBTO said Monday had gone offline in the days after the explosion, though it is in Russia's far east, a long way from the site of the accident. Peleduy had not previously been mentioned in relation to the blackouts.

The CTBTO did not immediately respond to a request for an update on the status of its Russian stations. Its website lists seven stations in operation in Russia, with another under construction. The two closest to the blast site went offline two days after the explosion, just before any particles from the blast are likely to have reached them, according to a simulation Zerbo posted.

Russian officials said the stations that went offline were having "communication and network issues," according to the CTBTO.

On Tuesday, Russian news agency Interfax quoted Deputy Russian Foreign Minister Sergei Ryabkov as saying his country's transmission of data from the radionuclide stations to the CTBTO was voluntary, and that the Aug. 8 accident was not a matter for the CTBTO anyway.

The Russian sensors are part of the CTBTO's International Monitoring System which comprises more than 300 seismic, hydroacoustic, infrasound and radionuclide stations dotted around the globe that together are aimed at detecting and locating a nuclear test anywhere.