The payment stems from a 2005 agreement between the government and Exxon Mobil, part of a broader push by Environmental Protection Agency to reduce air pollution from refineries, in which the company agreed pay $14.4m in civil penalties and for community-related environmental projects while also installing new air-pollution controls at the refineries.

Justice and EPA officials said Wednesday that Exxon Mobil violated the agreement by not adequately reducing smokestack sulfur pollution at the refineries as it had promised.

Exxon Mobil said in a statement that the company itself identified the continuing sulfur emission problem and brought the matter to the attention of the EPA.

"Environmental impacts associated with this item were very minor," Prem Nair, a spokeswoman for Exxon Mobil's offices in Fairfax, Virginia, wrote in an e-mail message. She said the emission problems have been corrected, and the company now meets the required EPA standard at the refineries in Beaumont and Baytown, Texas; Torrance, California, and Baton Rouge, Louisiana.

The Justice Department saw the matter as being a bit more serious.

"The Department of Justice will not tolerate violation of our consent decrees," Assistant Attorney General Ronald Tenpas said in a statement. "The significant penalty in this case shows that noncompliance with settlement requirements will have serious consequences."

Granta Y. Nakayama, assistant EPA administrator for enforcement and compliance, said the 2005 settlement "has resulted in major reductions in air emissions" from Exxon Mobil refineries, "but we need full compliance to realize all the benefits of the settlement."

The 2005 agreement was one of a number of settlements that covered companies and refineries across the industry. To date, the EPA said, 95 refineries in 28 states, accounting for 86 percent of the country's refining capacity have installed additional emission controls as part of the 2005 settlement or agreements patterned on it.

At the time, Exxon Mobil denied that it had violated any laws or clean air regulations but said the settlement was "in the best interest of the company and supports the continued trend of emission reductions" at its refineries.

As part of the settlement, Exxon Mobil originally paid a civil fine of $7.7 million, promised to spend another $6.7 million on community environmental improvements, and said it was installing additional pollution controls at six refineries. Its two refineries at Joliet, Illinois, and Billings, Montana, while part of the original settlement, were not involved in the latest penalty.

In October, Exxon Mobil broke its own record for biggest US quarterly profit, reporting earnings of $14.83 billion for the July-September period. The previous mark was Exxon Mobil's $11.68 billion profit in the second quarter of this year.