The Russian capital reported 7,704 new infections on Sunday, the most in a single day since Dec. 24. Authorities confirmed 14,723 cases nationwide, the largest one-day total since Feb. 13. The daily figures have remained high this week.
Mayor Sergei Sobyanin said Friday's caseload would be more than 9,000, which would be the most recorded in Moscow since the pandemic began. Until the beginning of June, daily new infections had been mostly below 3,000 for months.
"A new mutation has arrived, a new strain is active. It's more aggressive, it's harder to recover from, it spreads faster. It's much more likely to penetrate a person's immune system," Sobyanin said, according to the RIA news agency.
He made the remark as he met people who work in the Moscow restaurant business.
In separate comments at a government meeting broadcast on television, Sobyanin said the city was rapidly increasing its number of hospital beds to treat a possible influx of COVID-19 patients. There are currently enough beds available, he said.
"This dynamic is fairly unexpected given that more than 60% of Muscovites have either already been ill or been vaccinated - it is a large segment of the population... We of course did not expect an increase (in cases), but a decrease," Sobyanin said.
The head of the consumer health watchdog, who was also at the meeting, said the number of coronavirus cases attributed to the Delta variant, which was first identified in India, was rising significantly across the country.
Sobyanin did not say which variant he had been referring to. He announced no new restrictions to rein in the outbreak, but said they could be coming.
"We are very close to stricter decisions - temporary but stricter - in terms of restrictions," Sobyanin said at the meeting with restaurant business representatives.
The Kremlin has expressed dismay over the slow progress in Russia's vaccination programme despite Russia making its Sputnik shot widely available to people in Moscow in December.
Moscow authorities on Wednesday ordered workers with public facing roles to get vaccinated against COVID-19.
Sobyanin said the number of people signing up for the shot had tripled on Thursday, and that he expected the case surge to further spur demand: "I think the number will rise every day."