Russian will host the opening match for FIFA's Confederations Cup on Saturday in St Petersburg, a tournament widely seen as a dress rehearsal for the 2018 World Cup finals.
Volgograd, an industrial city 865 km (540 miles) south of Moscow, is not part of the Confederations Cup line up but will host four group-stage matches in the 2018 tournament.
Russia's Emergencies Ministry said the blaze, which was put out just before 7 a.m. (0400 GMT), had been caused by a "violation of fire safety regulations during welding".
No people were injured or killed in the fire, the ministry said in a statement.
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A spokeswoman for Stroytransgaz, the company building the stadium, told Reuters the fire was thought to have been caused by a welding spark that ignited thermal insulation materials before spreading to some plastic foam nearby.
The company — which is controlled by businessman Gennady Timchenko, a close ally of President Vladimir Putin who was slapped with U.S. sanctions after Russia annexed Crimea in 2014 — has yet to estimate the damage but expects it to be small, the spokeswoman said.
Video footage of the blaze circulated by Russian media showed a plume of thick smoke rising from the carcass of the 45,000-seat stadium.
"There was simply a lot of smoke, which made it look really bad, but no major damage," a source in the local World Cup organising committee told Reuters. "Some smoke damage to one of the columns but that can be painted over."
Russia will host the World Cup finals next year in 12 venues spread across 11 cities, including Moscow, Saint Petersburg, Sochi and Kazan.
These four cities are hosting the Confederations Cup, a two-week tournament that will feature the host country, world champions Germany and the various regional champions.
(Reporting by Moscow Newsroom; Editing by Alison Williams)
