The U.S. Department of Justice on Tuesday squared off Tuesday against two of the nation's largest airlines at the start of an anti-trust trial seeking to undo a deep collaboration between American Airlines and JetBlue.

The airlines in January 2021 formed the Northeast Alliance to share routes and customers in the U.S. northeast. The DOJ argues that the partnership will drive up ticket prices and give the partners an unfair competitive advantage.

The agreement between the carriers is a "de-facto merger," DOJ attorney William Jones II said Tuesday.

He added that the pact allows the pair to operate "like a single airline at the expense of travelers."

Last year, the DOJ, along with six states and the District of Columbia, sued the airlines, paving the way for the trial in Boston.

"In an industry where just four airlines control more than 80% of domestic air travel, American Airlines' 'alliance' with JetBlue is, in fact, an unprecedented maneuver to further consolidate the industry," U.S. Attorney General Merrick B. Garland said in a statement. "It would result in higher fares, fewer choices, and lower quality service if allowed to continue."

The DOJ is focusing on 29 non-stop routes where the airlines previously competed—11 from Boston and 18 from the New York City area.

The antitrust case against the airlines is just the latest in a series of such cases brought up by the DOJ in recent weeks.

"Capitalism without competition isn't capitalism. It's exploitation," President Biden wrote on Twitter Monday.

The White House Competition Council on Monday held its third meeting, where officials discussed policy on increasing competition in the economy and providing cost savings to families.