IDAHO CITY - A fire burning near Idaho City has grown to nearly 3,400 acres, prompting fire managers to expand the area closure around the fire.

The expanded closure on Saturday was due to the fire's recent spread and "long term potential to increase in size," officials said in a news release.

The Pioneer Fire was measured at 1,300 acres Thursday, but officials say wind shifts and changes in humidity helped it expand.

The closure northern boundary parallels the South Fork Payette River from Lowman to the Grimes Pass Road, but the river is open to rafting use. The new closure also borders Highway 21 from Lowman to nearly Idaho City, but the highway is open.

In addition, the new closure borders the Centerville and Grimes Pass roads, but those roads are also open.

A map and explanation of the revised closure can be found here.

The fire sparked on Monday in steep terrain about seven miles north of Idaho City. Nearly 400 firefighters are working the blaze, which was 26 percent contained on Saturday.

A community meeting was held Friday night at a community hall in Idaho City to update residents on the fire.

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Officials say no structures are currently threatened.

Investigators are still looking into what sparked the fire.