Five days after a brush fire began on the south slopes of the Big Island’s Mauna Kea volcano, the steep terrain and volcanic ash are causing problems for the 150 people trying to extinguish the blaze.

More than 1,387 acres have been scorched by the wildfire, which the Army says began sometime about 9:30 a.m. Sunday about a quarter-mile east of its Pohakuloa Training Area main gate in the vicinity of Mauna Kea State Park. No Army training activity was taking place on the east side of Saddle Road at that time.

Four helicopters have been pressed into service to fight the fire. No structures have been burned and no one has been injured.

Big Island police arrested a 31-year-old Papaikou man Monday on suspicion of first-degree arson. He was found by firefighters and taken to Hilo Medical Center, treated and released.

The Army said yesterday that the blaze is 60 percent contained.

The fire has damaged or destroyed power and communication lines that connect Kona and Hilo communities.

Bradshaw Army Air Field at PTA — which serves as an emergency field — has been closed because of the dense smoke.

Saddle Road, which connects East and West Hawaii, remains open with detours in place at mile markers 29 and 34.5.