Russian President Vladimir Putin has said that there’s no risk of increased radiation levels following a deadly blast at a military site in Northern Russia earlier this month that killed at least five people.

“There is no threat here, no increase in the [radiation] background exists either,” Putin said during a visit to France on Monday.

“We don’t see any serious changes there, but preventive measures are being taken so there are no surprises,” Putin continued, adding that independent experts were sent to the site to monitor the situation.

The explosion took place during a test of a liquid propulsion system at a military training ground in Russia’s northern Arkhangelsk region, about 30 kilometers from the city of Severodvinsk.

While the Russian military told state news agencies on the day of the blast that radiation levels in the vicinity were normal, Rosgidromet — a Russian meteorological agency — warned that it had recorded radiation levels four to 16 times higher than normal.

The explosion itself caused widespread international speculation that the accident involved a nuclear-powered cruise missile known as the Burevestnik or Skyfall.

The missile is believed to use a nuclear reactor to help power its flight, giving it the ability to fly for longer periods than a conventional missile.

While the Kremlin has confirmed that Russia is developing such a missile, it has not confirmed that it was involved in the failed test.

Earlier on Monday, CNN reported that four Russia-based nuclear monitoring stations that monitor radioactive particles in the atmosphere had mysteriously gone quiet after the August 8 explosion, according to the Comprehensive Nuclear-Test-Ban Treaty Organization, an independent body which watches for nuclear weapons testing violations with over 300 monitoring stations around the world.

Both Russia and the US are signatories to the treaty.

The two Russian radionuclide stations, called Dubna and Kirov, stopped transmitting data within two days of the explosion, the organization said. Two additional stations — in Bilibino and Zalesovo — went silent on August 13, a senior CTBTO official told CNN.

“Experts continue to reach out to our collaborators in Russia to resume station operations as expediently as possible,” the official said.

The organization has 80 radionuclide stations around the globe which “measure the atmosphere for radioactive particles,” it says, adding that “only these measurements can give a clear indication as to whether an explosion detected by the other methods was actually nuclear or not.”