Pheonix ? A fire burning in a national forest threatened two transmission lines that send electricity to metropolitan Phoenix, officials said Sunday.

Three planes dropped retardant on the fire, which was burning about 5 square miles but wasn’t threatening buildings, said Emily Garber, a spokeswoman for the Tonto National Forest near Sunflower. It was about two miles from the power lines, two of many that bring power into the Phoenix area.

Arizona Public Service, one of two major utilities serving the metro area, can use contingency plans to import power into Phoenix if a transmission line is lost, company spokesman Alan Bunnell said.

Garber said crews, which included 110 people and a water-dumping helicopter, were optimistic that retardant would keep the 3,500-acre fire from advancing toward the power lines. The fire, which was sparked by lightning Friday afternoon, was not contained.

Crews worked to clear away brush between the fire and the community of Sunflower, about five miles north of the fire and 55 miles northeast of downtown Phoenix.