(Ripley County, Ind.) - Weapon testing by the Air National Guard sparked a fire Tuesday at Big Oaks National Wildlife Refuge, formerly the Jefferson Proving Grounds, in Ripley County.

The U.S. Fish and Wildlife Service fire specialist Brian Winters said the blaze is mostly in the Ripley County portion of the preserve, with part of it reaching into Jennings County.

It started as a field fire as weapons testing was performed at the Air National Guard range inside the preserve.

Almost 500 acres have burned as of Tuesday evening with some areas still on fire as of Wednesday morning. Crews were using a 100 acre back-burn to limit the fire's spread.

Winters said the fire poses no threat to residences. There is heavy smoke in the area, especially along U.S. 421 and other nearby roads.

The fire is likely to smolder until a soaking rain comes in.

In 2000, about 50,000 acres of the former Jefferson Proving Ground in Ripley, Jefferson, and Jennings counties became the Big Oaks National Wildlife Refuge.

While this fire was unplanned, occasional prescribed burns are used to manage the grassland habitat, according to the U.S. Fish and Wildlife Service website.