John Legere, chief executive officer and president of T-Mobile US Inc., left, listens as Marcelo Claure, chief executive officer of Sprint Corp., speaks during an interview on the floor of the New York Stock Exchange (NYSE) in New York, U.S., on Monday, April 30, 2018.

The Federal Communications Commission needs more time to review the proposed Sprint-T-Mobile deal, the agency said in a letter to the companies Tuesday.

The agency has paused an "informal" 180-day transaction clock "to allow for thorough staff and third-party review" of recently submitted materials.

Sprint and T-Mobile have gone down a rocky road to a merger, calling off and resuming talks. The companies announced that they would merge last April in a bid to cut costs and combine forces to develop a next-generation network called 5G, which would provide faster speeds, more capacity and lower response times.

But the companies could encounter hurdles to gaining regulatory approval for the tie-up. A deal between T-Mobile and Sprint, who are the third largest and fourth largest wireless carriers in the United States by subscribers, previously faced opposition from antitrust regulators under President Barack Obama's administration.

Shares of Sprint and T-Mobile were essentially flat in extended trading.

Here's the full letter: