CFB PETAWAWA, Ont. - Firefighters from Canadian Forces Base Petawawa continue to battle a forest fire that is threatening to spread into Algonquin Park.

Campers have been put on alert that they may need to evacuate the area after a fire that started Thursday afternoon grew to 125 hectares on Friday.

By late afternoon Friday, the fire was within 300 metres of the east boundary of Algonquin Park.

"It's not a huge fire, but it currently is not under control. If the weather turns on us it could take a run," issue and information officer for Pembroke District of the Ontario Ministry of Natural Resources (MNR) Doug Skeggs said.

"We figure there (are) 300 to 400 people we need to be looking for to get information to," if an evacuation is needed, he said.

Officials took the unprecedented step of closing access to the east side of the province's largest park.

On Friday afternoon, barricades went up in front of the Algonquin Portage store, traditionally the last pit-stop for campers before heading into the park.

Laurie Strain of Toronto was canoeing with her family at High Falls, situated 10 kilometres west of the park's boundary, on Thursday when they spotted black smoke in the far distance.

"We figured it was far enough off but we really had no idea where it was," Strain said.

When they returned to their campsite at Achray, a park warden warned them that if they were leaving they should do it sooner rather than later.

"They were letting people in last night but the concern has grown since then," she said, adding the warden told her the park may be closed for 24 hours.

Cpt. Sally Ann Cyr, of the CFB Petawawa public affairs office said the cause of the fire was still unknown.