HOUSTON - The largest oil refinery in the United States released more than eight million pounds of illegal pollution in the last five years, violating the federal Clean Air Act thousands of times, according to a lawsuit filed Tuesday by environmental groups in Texas.

The lawsuit against ExxonMobil Corp. is the latest by Sierra Club and Environment Texas as part of their campaign to rein in what they call "illegal emissions" by dozens of refineries and chemical plants that operate in the Texas Gulf Coast. In recent months, the groups have reached multimillion-dollar, out-of-court settlements with Shell Oil Co. and Chevron Phillips after filing similar suits.

ExxonMobil denied the allegations and said it would fight the lawsuit.

In the last five years, the Exxon refinery in Baytown has produced annual emissions that are 40 percent lower than the limits set by the U.S. Environmental Protection Agency, company spokesman Kevin Allexon said in a statement. Since 2003, Exxon has invested nearly $1.3 billion in the Baytown facility to improve environmental performance, he added.

"ExxonMobil works hard to operate within regulatory standards while continuing to make significant improvements in its environmental performance through emissions controls, technology enhancements and process changes," Allexon said.

Texas has more oil refineries, chemical plants and coal-fired power plants than any other state and is the nation's leader in greenhouse gases. The state produces more than 20 percent of the nation's oil and one-third of the country's gasoline is refined along the Texas Gulf Coast.

The lawsuit, filed in federal court in Houston, accuses Exxon of violating emission limits on sulfur dioxide, a component of acid rain; hydrogen sulfide, a toxic, flammable gas characterized by a rotten egg smell; such cancer-causing agents as benzene and butadiene; carbon monoxide; and the smog-causing agent nitrogen oxide.

It says the "emissions events" are usually caused by equipment breakdowns and malfunctions. For example, in October, there were two such events in the same day, according to the lawsuit.

The legal maneuvers are part of broader accusations by the environmental groups and the U.S. Environmental Protection Agency that state regulators are not properly monitoring and enforcing federal emissions standards.

State regulators say their rules decrease pollution, but are not so stringent that it becomes too expensive to operate in the state.