MENDOCINO (CBS SF) — A 200-acre vegetation fire burning in Mendocino County was 20 percent contained on Monday evening, according to Cal Fire.

The blaze was reported Sunday at 5:51 p.m., and was burning southwest of Ukiah on Boonville Road and Highway 253.

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The rugged steep terrain has made it difficult for firefighters to access the area.

No structures were threatened and there have been no fatalities.

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According to fire officials, crews were working overnight to “strengthen containment and extinguish hotspots.”

Inmates from the California Department of Corrections are among the 73 personnel battling the fire.

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The cause remains under investigation.