A helicopter carrying four people crashed Thursday, sparking a wildfire in dry forest that prevented attempts to rescue possible survivors, authorities said.

All four on board were believed to have died, Kittitas County Undersheriff Clayton Myers said, but authorities could not confirm their fate because the fire was preventing them from reaching the helicopter.

"Fighting the fire is the priority right now, because it's preventing us from" reaching the helicopter and investigating the cause of the crash, Myers said.

Myers said the flight was believed to have originated from Boeing Field in Seattle.

The names of those on board were not immediately available.

A Federal Aviation Administration spokeswoman in the Seattle area said she had no information on the flight.

The helicopter crashed about three miles south of Easton on the east slope of the Cascades. The fire spanned an estimated 300 acres by Thursday night. No homes were immediately threatened, Myers said.