The US State Department has warned Americans to "carefully consider" the risks involved in travel to parts of Russia, as heavy smog continues to grip Moscow and wildfires rage.

"Forest fires and extreme high temperatures in the Moscow region and surrounding areas of central Russia have produced hazardous levels of air pollution and caused numerous flight delays and cancellations in Moscow," the department said in a warning that runs until September 5.

"The hazardous air quality means that persons with heart or lung disease, older adults, and children should remain indoors and keep activity levels low. Everyone should avoid prolonged or heavy exertion outdoors," it added.

"Persons contemplating travel to Moscow and surrounding areas should carefully consider their plans in light of these developments,” the document went on.

The U.K. Foreign Office has also advised “against all travel to specific regions of Russia, and against all but essential travel to other specific regions of Russia.”

Wildfires continue to rage across much of the central part of European Russia as the country experiences an unprecedented heat wave, with temperatures of up to and above 40 degrees Celsius (104 degrees Fahrenheit).

The death toll in the blazes rose to 52 on Friday.

MOSCOW, August 7 (RIA Novosti)