Russian authorities have ordered residents near the scene of a mysterious blast that killed five nuclear scientists and sent radiation levels soaring to leave, only to backtrack on their instructions within hours.

Secrecy surrounds the deadly rocket engine explosion at the Nyonoksa military site, which is about 30 kilometres from the north-western city of Severodvinsk.

Russia’s state weather service said radiation levels spiked in Severodvinsk by up to 16 times on August 8 after what officials say was an explosion during a rocket engine test on a sea platform.

“In six of the eight points in Severodvinsk, an excess gamma radiation dose rate at 4 to 16 times the background values was detected,” federal environment watchdog Roshydromet said.

State nuclear agency Rosatom said the experimental rocket’s fuel caught fire after a test, causing it to detonate.

Five of Russia’s best nuclear scientists were reported killed in the explosion, while three more Rosatom employees were injured.

Locals were initially urged only to take iodine tablets (used in treating radiation sickness) and stay indoors. Rosatom did not concede until two days later – after the spike in radiation levels was reported – that the accident involved nuclear materials.

Residents of Severodvinsk were initially urged to stay indoors after the blast. Photo: Getty

Authorities then recommended residents of Nyonoksa leave while repair work was carried out, Interfax news agency said, citing local officials.

“We have received a notification … about the planned activities of the military authorities. In this regard, residents of Nyonoksa were asked to leave the territory of the village from August 14,” authorities in Severodvinsk were quoted as saying.

But just hours later, Ksenia Yudina, a spokeswoman for the Severodvinsk regional administration, said state nuclear agency Rosatom had cancelled the planned activities and villagers could return to their homes.

Russian news agency Tass said on Tuesday that laboratory analysis of soil, water and sand samples had confirmed that radiation levels did not exceed background rates in Severodvinsk and adjacent areas, following the engine explosion.

Russian Academy of Sciences spokesman Leonid Bolshov earlier told Tass that there was no threat to human health of people after the incident at the military test site.

The five workers killed in the incident and three survivors have been honoured with Russia’s Order of Courage, state media reported.

-with AAP