Shares of Sprint and Nextel Communications rose after reports that the American cell phone companies are in merger talks.

The combined company would be worth about $70 billion, financial news network CNBC said, citing unidentified people familiar with the situation. The Wall Street Journal said the companies are discussing a “merger of equals.”

Pressure on cell phone companies to increase in size has grown since Cingular bought AT&T Wireless Services for $41.3 billion in October, becoming the biggest provider ahead of Verizon Wireless.

Sprint’s chief executive, Gary Forsee, and Nextel’s chief executive, Tim Donahue, would combine their companies into the third-largest provider, with 33 million subscribers.

A Nextel spokeswoman, Audrey Schaefer, declined to comment. A Sprint spokesman, Scott Stoffel, also declined to comment.

Shares of Sprint, which ranks third in the American market, rose $1.78, or 7.9%, to $24.28 in New York Stock Exchange composite trading. Nextel, the largest provider of push-to-talk mobile telephones, rose $1.84, or 6.6%, to $29.81. Verizon Wireless and Cingular shares don’t trade.

A merger of Sprint and Nextel would reduce the number of large American cell phone providers to four from five.

A combined Nextel, with 15.3 million subscribers, and Sprint, with 17.3 million, would also come closer to Cingular and Verizon in size. Cingular has more than 47 million subscribers and Verizon has more than 42 million.

By combining with Nextel, Overland Park, Kan.-based Sprint would gain access to a company that has the highest average revenue per user and the lowest turnover. Nextel’s push-to-talk wireless technology, with phones that act like walkie-talkies, is used mainly by businesses and contractors.

Sprint’s network would give Reston, Va.-based Nextel a head start in upgrading its existing airwaves to high-speed technology.

Nextel’s chief financial officer, Paul Saleh, said yesterday the company will announce by February its choice of suppliers to expand its wireless data network.