The acrid haze hindered visibility all over the city of three million people and the smell of smoke permeated homes as Kiev jumped to the top of high air pollution rankings.

A forest fire broke out almost two weeks ago close to the Chernobyl reactor that exploded in 1986 in the world's worst nuclear accident.

On Tuesday, Ukrainian authorities said the blaze had been largely extinguished thanks to heavy rain.

But on Friday a thousand firefighters aided by two planes and three helicopters were still battling a number of small blazes in the wooded area some 80 kilometres (50 miles) north of Kiev, the emergency service said.

The sky became clearer in the evening as Kiev dropped from the top of the list to seventh place in the ranking compiled by IQAir Group.

The smog has been blown in by strong winds from dozens of forest fires in surrounding regions, including from the nuclear exclusion zone around the Chernobyl power plant.

On Friday, Ukraine's interior minister Arsen Avakov and head of emergency service Mykola Chechotkin reassured President Volodymyr Zelensky that the smog does not pose a chemical or radiation threat.

Kiev authorities made similar assurances to residents in the capital.

"In Kiev, smoke and air pollution are being observed as a result of fires... but there is no radiation," Kiev Mayor Vitali Klitschko said in a video address.

Officials nevertheless urged locals not to go out without urgent need and to keep their windows shut.

The Kiev air pollution hit a city already under lockdown due to the coronavirus epidemic, with 4,662 confirmed cases in Ukraine and 125 deaths.

In Kiev, people are allowed to walk outside but only wearing masks and not in groups of more than two.