Russian troops dug a five-mile canal yesterday to protect a nuclear arms site from wildfires caused by a record heatwave.

The forest and peat fires have killed at least 52 people, made more than 4,000 homeless, diverted many flights and pushed air pollution in Moscow to six times its normal level, forcing some residents of the capital to wear surgical masks.

"The fire situation in the Moscow region is still tense, but there is no danger either for residential areas or for economic sites," an emergencies ministry spokesman said.

Weather forecasts said the smoke, which has reached even underground metro stations, would persist until Wednesday.

The canal was dug at Sarov, a closed town 220 miles east of Moscow, whose nuclear site, ringed by forest, produced the first Soviet atomic bomb in 1949 and remains Russia's main nuclear design and production facility.

The emergencies ministry said that the situation in Sarov had "stabilised", and Russia's nuclear chief assured President Dmitry Medvedev that all explosive and radioactive material had been removed from the nuclear site as a precautionary measure.

Russia, one of the world's top grain producers, has also brought in a temporary ban on exports after crops were ravaged by the dry weather. The news sent world wheat prices soaring.

The temperature climbed to 36C yesterday.