A vegetation fire was burning out of control at Samuel P. Taylor State Park in Marin County early Tuesday morning, consuming 100 acres across steep terrain and prompting officials to order evacuations.

The fire started under the Mt. Barnabe fire lookout before 8 p.m. Monday and was threatening 20 to 30 structures, according to Marin County Fire officials.

By 12:45 a.m., officials said they had the fire 10 percent contained and were making good progress in the firefight with bulldozers, hand crews, water tenders and engines.

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Still, Marin County fire officials ordered evacuations for “Mountain King Road, Portola Avenue and Alamo down to Barranca.” They advised residents to immediately evacuate to the Lagunitas School and wait for further information.

An evacuation order is in place for Mountain King Rd, Portola Ave and Alamo down to Barranca. Residents in that area should evacuate immediately to the Lagunitas School and wait for further information. Alamo Rd. will be closed until further notice. pic.twitter.com/0eqevGsb83 — Marin County Fire (@marincountyfire) September 11, 2018

Smoky conditions were reported in San Rafael, and residents in Fairfax and San Geronimo were advised that smoke could be especially strong in the area.

Marin County residents in the vicinity were encouraged to monitor local news media and follow any instructions form Marin County Fire or Sheriff officials.

For more information, firefighters set up a public information line: 415-473-7191.

Jill Tucker is a San Francisco Chronicle staff writer. Email: jtucker@sfchronicle.com Twitter: @jilltucker