'Arsonists' seen near Swinley Forest fire in Berkshire
Youths have been spotted setting fires in woodland as firefighters battled to control a forest blaze, Royal Berkshire Fire and Rescue Service said.
Flames have swept through an area of about 300 hectares of Swinley Forest, near Ascot, Berkshire, since Monday.
The fire service said the "sheer stupidity" of starting fires put lives in danger as about 100 firefighters tackled blazes for the third day.
A fire continues to burn close to the Devil's Highway near Bracknell.
The A3095 Crowthorne Road is closed in both directions due to the fire, which has also affected the B3430 Nine Mile Ride between Crowthorne and the Coral Reef roundabout.
Firefighters saw a group of teenagers running away after setting fire to woodland on Tuesday evening in the Crowthorne area of the forest.
"Luckily we have got fire crews all over the place," a fire service spokeswoman said.
"The sheer stupidity of it beggars belief and puts lives at risk."
It is not known how the earlier fires started and walkers have been warned to take care when discarding cigarettes.
Calm winds overnight stopped the fire from spreading, which allowed crews time to dampen down.
High-security Broadmoor Hospital is on the edge of the forest.
Station manager Paul Jones said: "We have to make sure that we don't push the fire in one direction where we could push it on to one of the houses, or the pub or the hospital or the main roads.
"The problem we have (is) there is still a lot of undergrowth which is still burning and there is still a lot which hasn't been burnt, so even if we don't extinguish it today, over the coming weeks and months if it carries on like this then we could find ourselves being called back."
Some nearby homes, which are made from wood, were evacuated as it was feared they could be caught up in the fire but residents have been allowed back to collect belongings.
Schools in nearby Bracknell and Crowthorne were closed on Tuesday but have since reopened.
Conservationists said they were concerned about the impact of the fire on wildlife and plants.
The 2,600-acre (1,052-hectare) forest contains conifer pine trees that are managed as timber crop and the woodland is also part of a Special Protection Area (SPA) for three rare birds - Dartford warblers, woodlarks and nightjars.
People have been warned to avoid the area.