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Tree-felling crews are at the scene of the 20-acre fire in the East Bay hills today to bring down trees that pose danger to firefighters battling the blaze in steep terrain, an Oakland fire spokesman said. 

The fire, which was first reported at 1:05 p.m. Wednesday near Grizzly Peak Boulevard and South Park Drive, is 50 percent contained as of late this morning. 

About half of the fire is within Oakland city limits and the other half is on land owned by the Lawrence Berkeley National Laboratory, fire officials said. 

Crews are working in "steep, challenging terrain" with large trees serving as fuel sources for flames, Oakland Fire Engineer Charleton Lightfoot said. 

"If the integrity of a tree has been compromised, the overall goal this morning is to remove those potential hazards," Lightfoot said. 

He said the goal by the end of the day is to have 100 percent containment and move to the "mop-up" phase of the fire. 

One Cal Fire firefighter suffered minor injuries after falling about 50 feet down steep terrain while battling the blaze, Lightfoot said. No other injuries have been reported. 

Grizzly Peak Boulevard will be closed between Centennial and South Park drives until at least 8 p.m. today, he said. 

Fire department officials are advising people to avoid the area to allow crews to access the area of the fire without any trouble.