Thousands of people on Monday attended a funeral for five Russian nuclear engineers killed by an explosion during tests of a new rocket at the country's main nuclear weapons test center.

The deadly explosion occurred on Thursday during a test of a liquid-propellant jet engine near the small town of Nyonoksa in Russia's northwestern Arkhangelsk region.

Initial reports said the explosion killed two people and injured a further six. Russia's Rosatom nuclear agency later acknowledged that the blast also killed five of its workers and injured three others.

Rosatom said the explosion occurred while the engineers were testing "a nuclear isotope power source" for a rocket. It named the five as Alexei Vyushin, Evgeny Koratayev, Vyacheslav Lipshev, Sergei Pichugin and Vladislav Yanovsky.

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A Russian state commission was investigating what went wrong, according to Valentin Kostyukov, head of the nuclear center, who called the five nuclear experts "national heroes."

Rosatom director Alexei Likhachev praised the victims as "true heroes" and "a pride of our country."

"Our further work on new weapons that we will certainly complete will be the best tribute to them," Likhachev said during the funeral, according to Rosatom. "We will fulfill the Motherland's orders and fully protect its security."

Sergei Kirienko, Russian President Vladimir Putin's deputy chief of staff, said the five would be posthumously awarded with top medals.

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Radiation spike

Russian officials at the test center where the scientists had worked said the accident had caused a two-fold rise in radiation levels that had only lasted an hour. The site is used to test intercontinental ballistic missiles that are used on nuclear submarines.

"One of the lines [of research and development] is the creation of sources of thermal or electric energy using radioactive materials, including fissile materials and radioisotope materials," said Vyacheslav Solovyev, an official at the test site.

Local authorities in nearby Severodvinsk reported a brief spike in radiation levels after the explosion.

The Soviet Union was the site of the world's worst nuclear disaster when, in 1986, an explosion at the Chernobyl nuclear plant in the former Soviet republic of Ukraine released radioactive contamination for nine days.

Read more: Explosion at Russian missile test site kills 2 people

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kw/aw (AP, AFP, dpa, Reuters)

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