The daily mortality rate in Moscow has nearly doubled amid a record heatwave, with hundreds of extra deaths each day compared to a normal period, the city's top health official said today.

"In usual times 360-380 people are dying each day. Now it is around 700," the head of the city's health department, said Andrei Seltsovsky, the RIA Novosti news agency reported.

"Our mortality rate has doubled," he added, saying that 1,300 places were currently occupied out of 1,500 spaces in city morgues.

His comments broke days of official silence over the effects of the unprecedented heatwave and smog from wildfires on mortality rates, amid growing evidence in the press of a sharp rise in deaths.

Seltsovsky said efforts to store the increased numbers of corpses were being complicated by the desire of many relatives to wait several days before their loved ones are laid to rest.

"Around 30 percent (of relatives of the dead) ask for their loved ones not to be buried on the second or third day (after death) but on the fourth or fifth day," he said.

"This, of course, complicates the situation somewhat," he added.

Meanwhile, deputy Moscow mayor Vladimir Resin said according to RIA Novosti that calls to municipal clinics had increased by some 20 percent amid the heatwave.

He said that clinics were now working 24 hours a day.

Officials said that the concentration of carbon monoxide in Moscow was still more than double acceptable safety norms as the smog from wildfires showed no sign of shifting from the Russian capital.