The Emergency Situations Ministry has warned Muscovites to keep their windows shut if they smell anything “chemical” in the air, as a mysterious stench has yet again been reported in the Russian capital.

Residents of several Moscow districts complained of a “strong unpleasant smell” on Sunday, the local branch of the ministry said in an online statement. Sokolniki, Lefortovo and Nagatinsky Zagon were among the districts to be worst affected by the stench, the city’s Moskva news site reported.

The Emergency Situations Ministry said environmental monitors were investigating the problem and that so far no threats to the public health had been identified. It nevertheless recommended that residents who noticed a “chemical smell” should stay indoors, keep their windows and doors shut, and seal any door cracks with wet towels.

The ministry warning comes after another mysterious stench enveloped Moscow in November, followed a month later by an abrupt surge in the levels of hydrogen sulfide levels recorded in the city, prompting similar warnings to stay indoors.

Officials blamed the November fumes on a problem at an oil refinery run by Gazprom. A Moscow court slapped the factory with a 250,000 ruble (then $4,500) fine for the pollution, according to the RAPSI legal news agency, though Gazprom denied that it was responsible for any increase in fumes.