One worker was unaccounted for and three had to be rescued by helicopter after a fire broke out in a 1,000-foot-tall smokestack under construction at an American Electric Power plant on Saturday.

The three contract workers were plucked off the stack by a helicopter about 10 p.m., after being trapped for about two hours above the flames, said AEP spokeswoman Carmen Prati-Miller.

The fire occurred at AEP's Kammer-Mitchell plant south of Moundsville, which is about 68 miles southwest of Pittsburgh.

Prati-Miller said the four workers were employed by Pullman Power, which has a contract to install a fiberglass lining in the stack. Apparently the liner caught fire about 8 p.m., but Prati-Miller said officials did not immediately know the cause.

The plant is undergoing an upgrade to bring the coal-fired power station into compliance with federal air pollution regulations.

Traffic on state Route 2 near the plant was blocked, she said.