UPDATE 1-Unlimited US mobile plans spark price war concerns

(Adds T-Mobile USA details, Verizon comment)

By Sinead Carew

NEW YORK, Feb 19 (Reuters) - AT&T Inc (T.N), Verizon Wireless and Deutsche Telekom's (DTEGn.DE) T-Mobile USA on Tuesday announced flat rate plans for unlimited calls in the United States, raising investor concerns that a price war could break out and sending shares lower.

Verizon Wireless, a venture of Verizon Communications Inc (VZ.N) and Vodafone Group Plc (VOD.L), said early on Tuesday it had started offering unlimited calls for $99.99 a month.

AT&T followed with a similar plan, while T-Mobile USA went a step further by including unlimited texts as well as calls for the same price.

While the new offers are not likely to make a noticeable dent in the operators' average service revenue, analysts said investors were worried that they would trigger further price cuts that would hurt profit and revenue.

The actions put pressure on Sprint Nextel Corp (S.N), the No. 3 U.S. wireless service, to match the plans or risk customers defecting, analysts said.

"I think this increases the potential for an all out pricing war," said Stanford Group analyst Michael Nelson.

He said the plans were likely to prompt high-spending customers to downgrade their plans rather than encourage customers to upgrade to unlimited plans.