The International Brotherhood of Teamsters on Wednesday expanded its lawsuit against the government in a long-running battle that has stopped Mexican trucks from coming deep into the United States.

In papers filed in federal appeals court in Washington, the union said the government must first assess the environmental impact of a pilot project before letting it continue. The first Mexican truck in the pilot program crossed the border last month.

Teamsters President James P. Hoffa said opening the border to the trucks is an attack on the environment, on highway safety and on American truckers and warehouse workers.

"It's outrageous enough that we've outsourced millions of jobs to foreign countries, but now we're bringing foreign workers here to take our jobs," Hoffa said in a statement. "This is another pressure the American middle-class doesn't need."

Under the North American Free Trade Agreement signed nearly two decades ago, trucks from both countries were supposed to have unhindered access to highways on either side of the border.

Mexico's Ambassador to the U.S., Arturo Sarukhan, said that the Teamsters are engaging in protectionism.

"First it was about the safety of Mexican rigs; now, with nowhere to run with that argument, the new red herring is an alleged environmental impact," Sarukhan said in a statement.