Residents of the city awoke on Wednesday to find the city enveloped, with the smell of burning hanging in the air.

Doctors have told people with heart conditions and other health problems not to venture out.

Even healthy people are being warned to avoid physical exertion.

...before really taking hold on Wednesday morning

"This smog is harmful for all people, especially for children and pregnant women," Roman Vinford, head of the Russian meteorological service, told the French news agency AFP.

Some of the city's famous landmarks were left shrouded in smoke.

"As the day broke, the skyline produced an illusion of gathering storm, without a single drop of rain ever falling on city streets," reported the Russian news agency Tass.

The dome of the Christ the Saviour Cathedral, on the banks of the Moscow River, was barely visible. Other landmarks, like the Novodevichy monastery, were reduced to eerie shadows.

The smoke is expected to linger until the weekend, when rain is forecast. The fires are not expected to threaten the city directly.

The situation is believed to be the worst to hit the city for decades.

Tinder-box

More than 100 forest fires covering more than 600 acres (243 hectares) are currently burning around Moscow, said the Russian news agency RIA-Novosti.

The Shatura region, east of Moscow, is said to be the worst-affected.

Troops could be deployed there to help with the situation, a Russian source told Itar-Tass.

The forest fires have been burning for days, helped by high summer temperatures which have created tinderbox conditions in woodland.

In some places, even peat bogs have started to burn, making the blazes hard to extinguish as they continue to smoulder underground long after the flames have been dealt with.

Around 800 firefighters, 240 vehicles and several helicopters are involved in the battle against the flames.

Other parts of Russia, including the Pacific Far East, have also been battling forest fires. Yakutiya, Kamchatka and Nizhniy Novgorod are among the areas hit.