The US Federal Communications Commission on Tuesday told Sprint and T-Mobile it will take more time than first expected to review a proposed merger of the telecommunications firms.

"We are pausing the commission's informal 180-day transaction shot clock in this proceeding," the FCC said in a letter to the companies.

The FCC said the extra time is needed to assess updated engineering and economic modeling information submitted in connection with the merger.

"The clock will remain stopped until the applicants have completed the record on which they intend to rely and a reasonable period of time has passed for.staff and third-party review," the FCC letter stated.

Sprint and T-Mobile in April announced a deal to form a new company and push development of a super-fast 5G network.

The two firms had previously called off merger talks after failing to clinch mutually agreeable terms.

5G or fifth-generation wireless communications networks would enable services such as remote surgery or driverless cars and allow customers to experience video and virtual reality with greater ease.

T-Mobile is one of the biggest earners for its parent Deutsche Telekom of Germany.

In 2017, it added 5.7 million net new customers compared with 8.2 million the previous year, but the hoped-for merger with Sprint fell through in November.

T-Mobile and Sprint—a subsidiary of Japan's SoftBank—are at present the third and fourth largest US wireless operators, respectively.

Together they have about 131 million subscribers, virtually matching second-ranked AT&T and posing stiff competition to market leader Verizon Communications.