By Sinead Carew

NEW YORK, Nov 11 (Reuters) - T-Mobile US Inc announced on Monday a stock offering that could bring it almost $2 billion for spectrum purchases.

T-Mobile, the No. 4 U.S. mobile service provider, said it would sell 66.15 million common shares, or roughly 9 percent of its existing shares outstanding, and that it could use the proceeds to buy airwaves to bolster its network capacity for wireless data services.

Deutsche Telekom, which owns 74 percent of T-Mobile, said on Twitter that its stake would be cut to 67 percent after the sale, but that it was not selling its shares.

However, T-Mobile shares fell almost 3 percent in after-market trading after it announced the sale, which would bring in $1.96 billion, based on T-Mobile's Monday closing share price of $26.97, and includes an offer of 6.6 million extra shares for underwriters.

T-Mobile raised concerns among some investors over dilution of the value of its shares when it said on Nov. 5 it could raise money for spectrum in an equity offer.

Chief Financial Officer Braxton Carter said in an interview that day that T-Mobile would be careful to avoid significant share dilution in an equity offering and that it could also opt to take advantage of low interest rates in a debt offering.

T-Mobile declined to comment on Monday on the size or the timing of the offering and which spectrum it might purchase.

New Street analyst Jonathan Chaplin said the timing of the offering was surprising as he had expected T-Mobile to wait until closer to the time of a government auction of spectrum licenses owned by broadcasters, slated for 2014 or later.

Chaplin said the timing may mean that T-Mobile wants to raise funds ahead of a spectrum auction, which is expected to go ahead in January with bidders including Dish Network and Sprint Corp.

"There is a chance that they may get involved in the LightSquared process, but this seems unlikely," Chaplin said.

LightSquared Inc, a wireless telecoms business, is in bankruptcy, and its spectrum assets are to be sold at auction, which is expected to take place in December.

So far, satellite TV provider Dish is the only company that has publicly expressed interest in the LightSquared spectrum.

T-Mobile said on Monday that it has 728.7 million shares outstanding, including Deutsche Telekom's stake. Other shareholders own roughly 190 million shares.

Many U.S. mobile operators have been looking to increase their spectrum holdings to boost their network capacity to support customers who are using more and more bandwidth-hungry data services.

T-Mobile has to push hard to regain ground after losing subscribers for four years. It started to turn the corner with subscriber growth in the second quarter but it needs more spectrum to keep competing well with much bigger rivals Sprint, AT&T Inc and Verizon Wireless .