Russia's President Dmitry Medvedev has interrupted his holiday to hold emergency talks on wildfires raging across central Russia.
The death toll from the disaster has reached 48, and seven regions are under a state of emergency.
Authorities are especially concerned about fires burning near a major nuclear research centre in Sarov.
Air pollution in Moscow has soared as the capital is blanketed in thick smoke from peat bog and forest fires.
Mr Medvedev, who was on his traditional summer break in the southern resort of Sochi, returned to the capital to chair an emergency meeting of the national security council on the crisis that was sparked by a heatwave.
On Wednesday morning 520 fires were still burning over an area of 188,525 ha (465,000 acres), the emergency ministry said.
In a 24-hour period, 403 new fires had been recorded and 293 had been extinguished, the ministry added.
Authorities said more than 2,000 firefighters were battling fires near the Sarov nuclear research facility, in the Nizhny Novgorod region.
The head of Russia's nuclear agency, who supervised the operation on the ground, said the situation was under control.
"Nothing is threatening the centre's facilities," Sergei Kiriyenko told the Interfax news agency.
Prime Minister Vladimir Putin met emergency services in the Voronezh region south of Moscow, one of the worst affected areas where hundreds of houses burnt down.
Thousands of people have lost their homes in 14 regions of Russia over the past few days.
About a fifth of Russia's grain crop has also been destroyed.
Many children are being evacuated from summer camps threatened by fires.
No let-up in the heatwave, which has seen record average temperatures, is expected in the next few days, with the Moscow area expected to hit about 38C (100F) this week.
'Out of control'
On Tuesday, Emergencies Minister Sergei Shoigu warned that the situation was dangerously unpredictable.
"In some places it is getting out of control and urgent firefighting manoeuvres are needed," he said.
About 155,000 people, including 124,000 emergency workers, and more than 20,000 units of machinery are currently being used to fight the fires across Russia, the minister said.
Mr Shoigu confirmed that many of the fires were caused by human negligence.
An eyewitness travelling in forested areas east of Moscow over the past week told the BBC she had seen smokers on a train flick smouldering cigarette butts from windows, and motorists likewise discarding them along roads.
The head of the ministry's crisis centre, Vladimir Stepanov, said municipal bodies in central Russia "must mobilise all their forces, not just sit and wait for fire brigades to arrive".
Prosecutors have opened a criminal case against national park officials in the Sverdlovsk region in the Urals, who stand accused of failing to extinguish fires in the area.
Peat bog fires outside Moscow have shrouded the capital in smog for several days. Doctors say the elderly and toddlers should wear gauze masks outdoors.
Elena Lezina, an expert at the Moscow state agency that monitors air quality, said pollution in the capital had surged four to 10 times above safe levels on Wednesday morning.
More famous for its bitterly cold winters, the country's European part normally enjoys short, warm summers.
However, this July was the hottest month on record, with Moscow, which sees an average high of 23C in the summer months, sweltering in 37.8C last Thursday.
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