Heavy smoke from a large forest fire in Serbia blanketed parts of Sofia and western Bulgaria on August 18, with strong winds carrying the plume across the border, Bulgarian authorities said, according to a report by BTA.

The Sofia municipality said it had received more than 40 reports within an hour about smoke, smog and unpleasant odours in different parts of the capital. Air-quality monitoring stations recorded a rise in fine particulate matter around midday.

There were no reports of major fires in Sofia or its immediate surroundings, the municipality said, citing the city's fire and civil protection authorities.

"The smoke was caused by a large forest fire on the territory of Serbia," the municipality said in a statement, adding that winds had carried the smoke cloud towards the Sofia Valley.

Authorities in the nearby municipality of Kostinbrod also warned of heavy smoke and advised residents to keep windows closed and avoid prolonged outdoor activity.

The municipality moved a mobile air-quality monitoring station to central Sofia to track pollution levels in real time. It advised residents to close windows and offices until the smoke cleared, while recommending that children, pregnant women, elderly people and those with respiratory or heart conditions limit outdoor activity.

Serbia is battling dozens of active wildfires amid high temperatures, drought and strong winds. Its Emergency Situations Agency said 45 forest fires were active across the country, although most had been brought under control.

One person has died in a wildfire near Smederevo, while more than 3,500 hectares have been destroyed by a fire in the Deliblato Sands region that began on August 5.

The Deliblato Sands, one of Europe's largest continental sandy areas, has been hit by intense heat and prolonged drought, conditions that have increased the risk of wildfires across Serbia and the wider Balkans.

A major fire there was brought under control last week, although fires have also been reported near Kraljevo, Zajecar, Knjazevac and on Stara Planina.

Serbia has also been grappling with historically low water levels on parts of the Danube, raising risks for river-borne fuel imports and putting pressure on energy production.