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Firefighters work an area of a three alarm vegetation firewhere a flare up occurs in Pittsburg, Calif., on Thursday, March 20, 2014. (Susan Tripp Pollard/Bay Area News Group)

PITTSBURG -- A three -alarm vegetation fire burned some 40 acres near the Delta shore Thursday afternoon, challenging fire crews who worked to get at flames through thick mud and marshland.

The fire was reported about 3:15 p.m. near West 10th Street and Willow Pass Road. The black smoke from the blaze could be seen from miles away, in Concord and Martinez.

No injuries were reported. Fire officials said that no structures were threatened, though the flames did move very close to several towers carrying high-tension power lines.

PG&E spokesman J.D. Guidi said that the utility's crews were on the scene, watching for damage to the transmission lines. It was unclear if any equipment had been burned, but Guidi said that fire crews had not yet requested that the power lines be de-energized, and there were no power outages related to the fire.

Firefighters stage stage at the edge of a three alarm vegetation fire which can be seen from Willow Pass Road near power lines in Pittsburg, Calif., on
Firefighters stage stage at the edge of a three alarm vegetation fire which can be seen from Willow Pass Road near power lines in Pittsburg, Calif., on Thursday, March 20, 2014. (Susan Tripp Pollard/Bay Area News Group)

Contra Costa Fire Inspector George Laing said the fire was contained around 7 p.m. Firefighters were working to knock down the northern side of the blaze, which was close to the Delta shore.

A CalFire helicopter was dropping mud on the fire. The blaze was "particularly difficult" because of its location in marshland, Laing said; one firefighter sunk into the mud past his knees as he tried to battle the flames.

Firefighters from the East Bay Regional Park District were using their off-road fire truck to assist with the blaze.

There was no information immediately released about a possible cause of the fire.