Russia’s Rosatom state nuclear agency admitted on Saturday that an explosion on Thursday during a rocket engine test was nuclear in nature, making it the likely cause of a reported spike in radiation in a nearby city in northern Russia.

According to the organization, five of its staff members were killed in an accident involving “isotopic power sources in a liquid propulsion system.”

Russia was not forthcoming with additional details, saying only that a test had been underway near the White Sea in the Arkhangelsk region of northern Russia when a rocket’s fuel caught fire after the test on a sea platform, causing it to detonate, throwing several people into the sea.

Three other people were wounded in the blast.

“Searches continued as long as there was hope to find survivors,” the Interfax news agency quoted Rosatom as saying.

Nuclear experts from the United States have said they believe Russia was testing an SSC-X-9 Skyfall cruise missile—also known as a 9M730 Burevestnik—which uses a form of nuclear propulsion. Russian President Vladimir Putin has bragged that these missiles, which are designed to evade U.S. missile defense systems, can reach anywhere on earth due to the small nuclear reactors powering them.

While Russia’s defense ministry initially said no radiation leakage had taken place, local officials from the nearby city of Severodvinsk, home to nearly 200,000 people, reported a brief spike in radiation. The city’s statement was suddenly taken down on Friday with no explanation.

Local residents reportedly began stocking up on iodine, an element known to help mitigate the effects of radiation exposure.

Greenpeace cited data from the Emergencies Ministry showing radiation levels at 20 times the normal level in Severodvinsk, according to a report by Reuters, while The New York Times reported that Russian news media said radiation briefly rose to 200 times the normal levels.

Russian media indicated that the test may have taken place at the Nenoska Missile Test Site, frequently used for ballistic and cruise missile testing by the Russian navy.