The US Department of Justice (DoJ) wants to review T-Mobile's proposed $4.4 billion acquisition for a big chunk of UScellular's business.

In a filing to the Federal Communications Commission (FCC), the DoJ stated that it wants to do a review to determine if the transaction poses a threat to the country's national security.

Although T-Mobile is one of the big three US carriers, its parent company is German telco Deutsche Telekom.

DT regained majority control of T-Mobile last year and owns a stake of around 50.2 percent stake in the carrier.

In the letter to the FCC, the DoJ asked the regulator to carry out a review to "determine whether these applications pose a risk to the national security or law enforcement interests of the United States."

"US DoJ believes that such risk may be raised by the foreign participation (including the foreign relationships and ownership) associated with the applications, and a review by the Committee is necessary to assess and make an appropriate recommendation as to how the Commission should adjudicate these applications," it said.

"As with any application that is referred to the Committee, US DoJ further requests that the Commission defer action on these applications until the Committee has concluded its review and offered its recommendation."

In May, T-Mobile announced it had agreed to scoop up UScellular's operations, including its customer base and around 30 percent of the company's wireless spectrum.

UScellular will retain ownership of its other spectrum plus its towers, with T-Mobile entering into a long-term arrangement to lease space on at least 2,100 additional towers being retained.

UScellular is one of the biggest regional providers in the US, outside of the three traditional big carriers (T-Mobile, Verizon, and AT&T).

The company, which has around 4.5 million customers and operates in 21 US states, agreed last month to sell a portion of its spectrum to Verizon for $1bn.

Providing an update on the deal from last month, T-Mobile said the transaction is progressing and expects it to be completed next year. UScellular has previously told the FCC that the deal is essential to ensure the company's future.