Forest wardens today stepped up patrols in the Chernobyl fallout zone as a leading ecologist warned that fires could send radioactive particles as far as Moscow.

Around 160,000 emergency personnel are battling 600 wildfires across Russia, 290 of which ignited in the last 24 hours.

Greenpeace said at least 20 fires – three of them in a highly contaminated forest area – had broken out in the Bryansk region, bordering northern Ukraine, in recent days.

Bryansk was part of the zone sprayed with a plume of radioactive isotopes caesium-137 and strontium-90 when the Chernobyl power plant's fourth reactor exploded in 1986.

Alexei Yablokov, a member of the Academy of Sciences, warned that winds could spread contaminants embedded in trees and plants as they succumbed to the inferno.

"Radionuclides may reach places at distances of hundreds of kilometres, depending on the weather," he said. "If the Bryansk region is in flames, they can reach the Novgorod region, Moscow and, in some conditions, eastern Europe."

There were conflicting reports over the extent of the fires in Bryansk. Asked about the gravity of the threat, Gennady Onishchenko, the country's leading public health official, said: "There's no need to sow panic. Everything is quiet there."

But Russia's forestry protection service said it was increasing patrols in the area after around 30 hectares of land went up in flames.

"The situation is complicated, but stable and controllable," an official from the service told Interfax.

Greenpeace played down fears of Chernobyl pollution reaching Moscow, but said the harmful potential of smaller doses of radiation, combined with smog, carbon monoxide and other particles, should not be overlooked.

A veil of smog lifted from Moscow but temperatures remained high as political repercussions of the crisis emerged. It appeared that the absence of the city's powerful mayor during its hour of need could hasten his demise.

Yury Luzhkov left for holidays and "treatment for a serious sports injury" as the city sweltered on 2 August and did not return until Sunday, several days after a toxic cloud had enveloped Moscow.

A senior health official has said the smog killed at least 320 more people each day than usually die in the city.

Luzhkov, in office since 1992, is the last of the regional heavyweights in Russian politics, but his future as city boss has looked increasingly fragile amid allegations of sleaze and incompetence.

The prime minister, Vladimir Putin, greeted the tanned-looking mayor in a televised meeting yesterday, saying: "You were quite right to return from your vacation. Your timing is perfect."

Observers interpreted the comments as disapproval. "Luzhkov underestimated the political situation and he underestimated how serious and tense the situation in Moscow is," said Gleb Pavlovsky, a political analyst with close ties to the Kremlin.

"Surely, he is in a very weak position now and worsened it even more by saying, amid all that is happening, that the situation in Moscow is quite normal."

Before Luzhkov returned, his spokesman, Sergei Tsoi, had said there was little reason to cut short the break because the fires causing Moscow's smog were outside the capital and therefore "nothing depends on the city authorities in dealing with the current environmental situation".

Luzhkov, 73, denied rumours that he was getting treatment in Tyrol, Austria, but declined to say where he had been.

The deputy mayor, Vladimir Resin, made a clumsy attempt to exonerate his boss, saying he had a backlog of 370 days' holiday. "He could have taken a whole year off," he said.

But a Kremlin source said it was "too bad" Luzhkov hadn't returned sooner.

"The mayor's absence obviously did not help the necessary decisions to have been made in timely fashion," the source said.