Hot, dry, windy conditions helped a forest fire burning Friday in the backcountry of Rocky Mountain National Park to jump to roughly 100 acres. A growing contingent of firefighters, aided by helicopter and air-tanker support, were battling the flames.

The blaze, burning in a lodgepole forest, was spotted Thursday about 1 mile south of Mount Dickinson near the northern edge of the park. At the time it was estimated to cover 3-5 acres, but overnight it grew to an estimated 10-12 acres, and by Friday afternoon its size was estimated between 85 and 100 acres and expected to grow, according to park officials, who couldn't say how the fire started.

The area, about 7 miles from the nearest road, is heavily timbered, and some of the trees had been killed by mountain pine beetle infestations, which have plagued other parts of Rocky Mountain.

On Thursday aerial tankers dumped five loads of retardant on the fire, and eight smokejumpers parachuted to the fire lines late in the afternoon. A ground crew joined them later in the evening, the park said.

By Friday afternoon 20 crew members from the Roosevelt Hotshots with the U.S. Forest Service reached the area on foot. A medium-size helicopter was expected to reach the area Friday afternoon, and fire bosses called in a larger helicopter for water drops and requested two additional air tankers.

All designated trails in Rocky Mountain National Park remained open Friday afternoon, the park reported, although due to aircraft and firefighters working in the West Creek and Fox Creek drainages off-trail travel was not being allowed in the area south of the North Fork of the Big Thompson River, west of the North Boundary Trail, or north of Cow Creek and east of Mummy Mountain and Mount Dunraven. This closure area is a remote section of the park and has very low visitation.