William Baer, assistant attorney general for the US antitrust division is reported to hold the view that any merger in the market will be detrimental to competition.
The announcement follows rumours that SoftBank-owned Sprint and Deutsche Telekom’s T-Mobile US are in talks over a possible tie up between the two companies.
"It's going to be hard for someone to make a persuasive case that reducing four firms to three is actually going to improve competition for the benefit of American consumers," Baer said.
The US justice department blocked a proposed merger between AT&T and T-Mobile in 2011, which in turn led to more favourable competitive conditions for consumers, Baer added.
