The request may indicate that satellite television provider Dish, controlled by billionaire founder Charlie Ergen, is gearing up for a fight with Sprint over its plan to sell a 70 percent stake to SoftBank for $20 billion. Dish declined further comment on the matter.

Dish, which recently gained regulatory approval to build its own wireless service, told the Federal Communications Commission in a document dated December 20 that it wants a three-week extension to the FCC's January 4 filing deadline for petitions against the Softbank deal, which was announced in October.

Earlier this week, Sprint, which owns 50.45 percent of Clearwire, said it agreed to buy the rest of Clearwire for $2.2 billion, in a deal that would be conditional on the success of the SoftBank purchase.

Sprint, the No. 3 U.S. mobile provider, declined to comment on the Dish filing on Friday.

It sent the FCC an amendment to its application for approval of the SoftBank deal on Thursday including notice of its agreement with Clearwire, which would gives Sprint control of the smaller company's substantial spectrum holdings.

Dish said in its filing that Sprint's plan "raises a number of issues deserving of careful consideration" and that interested parties need an appropriate amount of time to consider and address these issues.

For example, Dish questioned if it is in the public interest for a foreign company such as SoftBank to control more wireless spectrum than any other company in the United States.

It also asked whether the FCC should re-evaluate "the competitive effects" of a combination of Sprint's and Clearwire's spectrum holdings under one owner.

Dish and Sprint recently clashed with each other during the regulatory review of Dish's plans for its spectrum holdings.

Sprint is already meeting objections to the deal from some minority shareholders who are not happy with the $2.97 per share price it agreed on with Clearwire.

(Reporting By Sinead Carew and Liana B. Baker; Editing by Leslie Adler)

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