UPDATE 4-Wal-Mart's takes discount wireless plans nationwide

* U.S. Wal-Mart stores to expand Straight Talk nationwide

* Seen sparking further mobile service price wars

* Offering developed with TracFone Wireless

* Leap to offer rival monthly plan in next several days

* Leap, MetroPCS shares dip (Recasts top paragraphs to emphasize competitive impact; adds analyst comment on T-Mobile USA)

By Nicole Maestri and John Tilak

SAN FRANCISCO/BANGALORE, Oct 14 (Reuters) - Wal-Mart Stores Inc (WMT.N) will sell cut-price mobile services nationwide, threatening to exacerbate a price war and hurt profit margins across the U.S. wireless sector.

The move pits Wal-Mart, teamed with a company owned by Mexican billionaire Carlos Slim, against low-cost carriers such as Leap Wireless International Inc (LEAP.O) and MetroPCS (PCS.N) and Sprint Nextel's (S.N) Boost Mobile.

T-Mobile USA, owned by Deutsche Telekom (DTEGn.DE), is also expected to bring its own cut-price service nationwide in coming weeks, adding further heat to the market.

"It raises the risk of an industry-wide pricing war," said Soleil/Nelson Alpha Research analyst Michael Nelson.

Shares of Leap and MetroPCS fell after Wal-Mart announced on Wednesday the expansion to more than 3,200 stores of the service, developed with TracFone Wireless Inc, the U.S. unit of Mexican cellphone giant America Movil (AMXL.MX) (AMX.N).

The TracFone offering includes a $30 prepaid plan for 1,000 minutes as well as a $45 per month plan for unlimited calling. The service runs on the network of U.S. mobile market leader Verizon Wireless, a venture of Verizon Communications (VZ.N) and Vodafone Group Plc (VOD.L).

Wal-Mart said it will be offering the plans nationwide on Oct. 18 after a pilot program in 234 stores this summer. It said that customer response to the program was "overwhelming."

While many investors had expected TracFone to expand its trial across the country, the announcement underlined increasing competition for Leap, MetroPCS and others.

"This clearly represents increased competition for the entire wireless sector and in particular for the unlimited pay-in-advance segment, of which Leap and MetroPCS are really likely to be negatively impacted the most," said Nelson referring to the Wal-Mart offering.

The retailer's offering could also rob customers from the No. 4 U.S. mobile service, T-Mobile USA, which has been losing ground to bigger rivals Verizon and AT&T Inc (T.N) as well as its smaller competitors Leap and MetroPCS.