The Department of Justice has begun looking into whether large U.S. telecommunications companies such as AT&T Inc. and Verizon Communications Inc. are abusing the market power they have amassed in recent years, according to people familiar with the matter.

The review, while in its early stages, is an indication of the Obama administration's aggressive stance on antitrust enforcement. The Justice Department's antitrust chief, Christine Varney, has said she wants to reassert the government's role in policing monopolistic and...