A range fire was reportedly burning Tuesday afternoon on the Yakima Training Center grounds.
Incident commanders said they hoped the blaze would calm down in the evening.
Military crews called in help from Yakima, Union Gap and East Valley, but few details were available because communication is difficult in the rough terrain.
The base commander, Lt. Col. Leo Pullar, said crews were still working to extinguish the flames late in the afternoon. The fire was burning in the southeast quarter of the training center.
Pullar said firefighters had been in that area before the fire, but he did not want to speculate on the cause of the fire before an investigation was complete.
Pullar said the fire attack was hampered by the rough, steep terrain and limited access.