Cantonal police said on Thursday that several army firefighting helicopters had been in use since Wednesday morning to tackle the flames covering a wooded area between the villages of Mesocco and Soazza.
The army is being supported by civilian firms.
The blaze was reported shortly before 6pm on Tuesday evening.
“Owing to the strong winds the fire has spread from an area 200 metres by 600 metres to an area 600 metres by 1,200 metres,” the police statement said.
The H13 main road has been closed off owing to the danger of rockfalls caused by the fire.
However, the A13 motorway, which was closed on Tuesday, reopened on Wednesday afternoon.
Four people were evacuated from houses in Mesocco, around 500 metres from the fire.
“Depending on how the situation develops, there could be further evacuations or road closures,” the statement said.
Meanwhile, another forest fire has broken out in the Calancatal valley, north of Braggio, police said.
The fire, which began on Wednesday night and spread over a 200 by 300-metre area, is now under control.
Despite the fact the fire came within 50 metres of a settlement, no evacuations were needed, the police statement said.
The cause of the fire is as yet unknown.
Elsewhere, Some 29 people were evacuated by helicopter in the southern canton of Ticino after another forest fire broke out on Tuesday.
Ticino police said that fire, in the Monte di Doro region, was started by four adolescents who lit a campfire.