U.S space agency NASA will not send their specialists to Russia for working on ISS mission's program until the acrid smog in Moscow dissolves, the deputy head of NASA's Human Space Flight Program in Russia (HSFP-R) said on Saturday.

"We have decided our colleagues should better stay in Houston [NASA headquarters is based there] until the Moscow weather changes," Mark Bowman said, adding the urgent trips related to the mission support will be carried out as usual.

Moscow has been hit by smog from nearby peat bog and forest fires mixed with pollutants for much of the last 12 days.

The smog has seen levels of carbon monoxide reached over 6 times the safe level. The fires have been caused by a heat wave that has seen temperatures more than 40 degrees Celsius (104 degrees Fahrenheit).

Bowman said the air filtration systems in the buildings that kept the interiors relatively smoke free, now had been overwhelmed forcing people to breathe contaminated air.

On Thursday, NASA satellites Terra and Aqua registered 535 hotspots from wildfires across Russia.

Earlier on Saturday the U.S. 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.

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

MOSCOW, August 7 (RIA Novosti)