Moscow's hospitals and ambulance service are working at peak capacity after a sharp rise in those hospitalised with serious coronavirus complications, a senior city official said Friday.

The densely populated capital with more than 12 million residents has the largest outbreak in Russia, with 7,822 confirmed coronavirus cases out of a national total of almost 12,000.

The number in Moscow's hospitals has doubled since last week and more than 85 percent of these patients have pneumonia, deputy mayor Anastasia Rakova was quoted as saying on the city virus task force's Telegram account.

Moscow is not sending people with mild symptoms to hospitals. Those who test positive are monitored through online video consultations. "Along with the growing numbers of people who are seriously ill, pressure has grown sharply on the capital's health service. Our inpatient facilities and ambulance service are now working at their limits," Rakova said.

Moscow has been under a lockdown since March 30 with residents only allowed to leave their homes for essential work, to buy food and medicine or to walk dogs within a radius of 100 metres. Driving private cars is permitted, however.

Meanwhile Mayor Sergei Sobyanin warned that the epidemic was not even close to its peak. "I can tell you for sure: there hasn't been any peak yet. We are more in the foothills of this peak, not even in the middle," Sobyanin, who is also the head of a taskforce coordinating regional responses, told the RIA Novosti news agency.

He said Russia had gained time with its anti-virus measures and was two to four weeks behind on the curve compared with countries suffering major epidemics. "We have that time lag so we can prepare better for the blow that is coming our way," the mayor said.

Moscow is rushing to construct a new virus hospital with 500 beds from scratch in a few weeks. It is due to open this month.