The Justice Department on Thursday approved the merger of NYSE Euronext and Deutsche Börse AG, which will create the world's largest stock exchange operator. The deal is still subject to the review of European antitrust authorities who have resisted approval.

"We are very pleased to have received the approval of the DOJ, an important milestone on our path to completing our compelling Trans-Atlantic combination," Duncan Niederauer, CEO at NYSE Euronext, said in a statement.

U.S. regulator's approval came with one condition: the German conglomerate has to sell its 31.54 percent stake in Direct Edge Holdings LLC, the fourth-largest U.S. exchange operator. The parties will have at least two years from the date of closing their combination to complete this sale.

"Without the divestiture and other restrictions obtained by the Justice Department, a combined NYSE and Deutsche Börse entity could influence the actions of Direct Edge, and thereby lessen the zeal of an aggressive and innovative exchange competitor," Sharis Pozen, acting assistant attorney general in charge of the DOJ's antitrust division, said in a statement.

The completion of the transaction is subject to further closing conditions, including approvals by the European Commission and other relevant national stock exchange supervisory authorities.