A foul odour has enveloped the Russian capital, with concerned Muscovites flooding authorities with complaints and officials scrambling to locate the source of the stench.

By Monday evening, the stench eased as authorities finally pinpointed the culprit: faulty air filters at a Moscow refinery, an emergencies ministry spokesman said.

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Residents in the city of some 12 million began complaining early on Monday of the “unpleasant smell of sulphur, hydrogen sulfide and oil products in the air,” Mosecomonitoring, Moscow’s environment watchdog, said in a statement.

The concentration of hydrogen sulfide in the air briefly exceeded the norm while the sulphur dioxide was within limits, the watchdog said.

Authorities warned against panic.

“The smell of hydrogen sulfide is not dangerous,” the emergencies ministry spokesman told AFP.

Amid the worst heatwave in its history, noxious smoke enveloped Moscow in the summer of 2010 as fires burned outside the Russian capital.

The smoke crept into apartments, offices, stores and even underground into the Moscow metro, forcing Russians to wear surgical masks or flee the debilitating combination of smog and high temperatures altogether.