The national emergencies ministry said that more than 500 fires covering just under 65,000 hectares of land were still ablaze across Russia, down 15,000 hectares from the day earlier.

US aid

Kormilitsyn said that 2,600 people and 200 pieces of equipment were being used to extinguish the fire, currently 17km southeast of Sarov.

"At the current time, efforts are being made to contain the fire within its existing limits and localise it," Kormilitsyn said.

A second fire in the village of Pushta, 21km southwest of Sarov, is smaller at 200 hectares and has been localised, the ministry added.

Officials emphasised that no fires had been recorded on the nuclear site itself.

In a phone call on Friday with his Russian counterpart Dmitry Medvedev, Barack Obama, the US president, said that the US development agency, USAID, the state department, the forest service and the state of California were mobilising firefighting equipment to help Russia.

Deaths queried

In Moscow, the first significant rain for weeks poured down overnight, with the capital enjoying much lower temperatures of up to 30 degrees Celsius.

There was little sign of the smog from the wildfires that had blighted the city in the last week but new reports emerged accusing the authorities of hiding the true death toll from the heatwave.

Moscow's top health official has already said the mortality rate had doubled in the heatwave, with hundreds more deaths every day than in usual periods.

However, the federal authorities have refused to confirm these figures.

Russia's Interfax news agency quoted Moscow doctors as saying they had been forbidden to give "heatstroke" as a cause of death to keep a lid on the statistics.

"We received the order not to use the diagnosis 'heatstroke'. We are told that the statistics for heatstroke were mounting up," one doctor told the news agency.

Contamination fear

Interfax said the heatwave had also forced Russia to stop producing biometric passports for a week after condensation infiltrated the server at the unit where they are produced when its air conditioning broke down.

Medvedev said that one quarter of Russia's crops have been lost to the fires [AFP]

Production has now resumed and the database was not damaged, the agency quoted the federal migration agency as saying.

With the full impact of the drought and fires becoming clear, Medvedev said on Thursday that one quarter of Russia's crops had been lost and many farms were now on the verge on bankruptcy.

As the authorities fight the blazes around Sarov, there have also been fears the fires could stir up particles on land in western Russia still contaminated by the 1986 Chernobyl nuclear disaster.

Sergei Kiriyenko, the head of Rosatom, Russia's nuclear agency, said there was no sign of abnormal radiation anywhere in the country.

"The radiation is at the usual background level," he said.

Fires have also blazed in neighbouring Ukraine, with the emergency services working to put out a two-hectare peat bog fire 60km from Chernobyl.