Nearly 250 people have fled from a 4-square-mile area because of poisonous carbon monoxide that has been seeping from an underground coal mine fire.

The Mathies Coal Co. mine near Finleyville, southwest of Pittsburgh, has been on fire since Wednesday. Evacuations began shortly after the blaze was discovered and were stepped up Friday night and early Saturday, officials said.Stagnant air caused more of the odorless gas to linger near the mine, said Washington County emergency services spokesman Phil Garrow.

"I'd compare it to standing right behind a bus in downtown Pittsburgh," Garrow said. "When it was nice and blowing earlier, it was great. It carried the stuff away. That has ceased."

About 25 people were evacuated late Friday when air quality declined dramatically, and 165 more were moved overnight. Another 50 residents of the Washington County community had left their homes since the underground fire broke out.

Carbon monoxide levels jumped from 3 to 6 parts per million Friday afternoon to 177 parts per million later in the evening, Garrow said.

Earlier Friday, crews began sealing entrances to the mine in an effort to extinguish the underground fire. National Steel decided to give up on pumping water into the ground.

"We're going to try to cut off all the air," National Steel spokesman Robert Toothman said.