Six US senators have written to the Justice Department and the Federal Communications Commission (FCC) with objections to T-Mobile's proposed acquisition of UScellular's wireless spectrum.

As reported by CNBC, a group of Democratic senators, including Bernie Sanders, have asked the Justice Department and the FCC to scrutinize the deal.

In May, T-Mobile agreed to acquire around 30 percent of UScellular's wireless spectrum for around $4.4 billion.

UScellular, one of the biggest regional cell carriers outside the big three of AT&T, Verizon, and T-Mobile, will retain 70 percent of its wireless spectrum and towers, leasing space on other towers to T-Mobile.

The regional carrier serves four million mobiles across 21 US states, in mostly rural areas.

Senators, however, fear the deal will negatively impact competition.

“Additional consolidation in the market would have far-reaching effects, reducing choices for consumers, further concentrating wireless spectrum holdings, and potentially leading to higher prices and other harms for consumers across the country,” the senators wrote in the letter, which CNBC said it had exclusively obtained.

The letter was led by Elizabeth Warren and Amy Klobuchar and was co-signed by Chris Murphy, Bernie Sanders, Cory Booker, and Richard Blumenthal.

T-Mobile said the agreement will expand its coverage across rural areas, which it notes are often underserved by the bigger carriers.

Both T-Mobile and UScellular have said that a deal will positively impact consumers.

The transaction is expected to close midway through next year, but still requires regulatory approval.