By Agence France-Presse
Washington ordered the shutdown of the facility along with two others on Thursday in a retaliatory move after the Kremlin demanded the US slash staff numbers at its Russian diplomatic missions.
Firefighters were called to the scene in response to alarmed -- or intrigued -- phone calls from citizens, but later clarified there was no cause for concern.
"The Russian embassy had a fire alarm NOT A FIRE everything is okay and we are clearing," San Francisco Fire tweeted from its official account.
Spokeswoman Mindy Talmadge told AFP that consulate employees "must be" in the middle of burning unknown items.
Firefighters "confirmed there was smoke coming from the chimney but no structural hazard," she added, without saying whether they entered the building.
The move triggered fevered speculation on Twitter about the nature of the material that had gone up in the flames.
"So they had so much to burn up that they set off their alarms with the smoke? Hahaha" wrote one user, while another said: "Much easier to burn than shred."
Another referenced ongoing official investigations into whether President Donald Trump colluded with Russia to help get himself elected: "Must be burning a big stack of Trump's love letters to Vladimir!"
US-Russia ties have slumped to their lowest point since the Cold War following the Kremlin's seizure of Crimea from Ukraine in 2014.
The West slapped punishing sanctions on Russia over its meddling in its ex-Soviet neighbor, sparking a revenge embargo from Moscow against agricultural products.