A wildfire has spread across at least a one-kilometre front on Ben Lomond this afternoon.

Firefighters were called to a western part of the Munro around 2.15pm on Tuesday. 

The National Trust for Scotland issued a plea to hillwalkers to avoid the area and have been warned there is no way down through the Ptarmigan path.

A statement from the governing body of protected sites warned that the blaze appears to have been caused by "a single dropped cigarette sometime in the last couple of hours". 

READ MORE: Scottish Fire and Rescue Service warns of 'extreme' wildfire risk in Scotland

"It is burning through an area where natural regeneration of woodland was just starting to take place," the statement added.

The Scottish Fire and Rescue Service confirmed they are in attendance with crews assessing the best way to extinguish the blaze on "difficult terrain".

A video shared on social media showed large clouds of smoke rising from the slope.

Ross MacDonald was hiking up Ben Ime and Ben Arthur when he saw the smoke from the blaze. 

Upon descent, he said the fire had already burned "a significant amount of land".

He told the Herald: "When we stated the ascent, I noticed smoke rising from the side of Ben Lomond.

"It was only once on the way back on the opposite side of the Loch that I realised the fire has burnt black a significant amount of land and continuing to move.

"The smoke had now become quite substantial."

The hillwalker said it was difficult to tell how serious it was as he had spotted the smoke from over seven miles away. 

"However, once closer I realised it was much bigger than expected and that the fire was still moving across the land," he added.

Ben Lomond is located on the east shore of Loch Lomond and with a peak at 974 metres it is Scotland's most southern Munro. 

A Scottish Fire and Rescue Service spokesperson said: "We were alerted at 2.15pm on Tuesday, 22 March to reports of a fire affecting grass on the west slope of Ben Lomond.

"Operations Control mobilised two appliances to the nearest roadside location to the fire at Rowardennan.

"Crews are currently monitoring the fire and assessing best options to extinguish it in difficult terrain."