Microsoft Corp is set to announce its long-awaited Project Pink phones early next week, sources familiar with the matter said on Monday, as the world's largest software company attempts to gain traction in the growing market for young smartphone users.

The mobile phones, to be sold by top U.S. mobile operator Verizon Wireless, are being targeted at heavy users of social network sites, according to sources. They will have a different name when the launch is officially announced.

In photos leaked to a tech blog last month, the new phones appear to be stylish, updated versions of Microsoft's Sidekick device, which is popular with the youth market.

The new phones, which likely won't be on the market until summer, are to be made by Sidekick manufacturer Sharp Corp, sources said.

Microsoft has sent invitations to media to attend an event in San Francisco next Monday, but declined to comment further. A representative of Verizon Wireless, a joint venture between U.S. phone company Verizon Communications Inc and Vodafone Group plc, also declined comment.

The new phone does not appear to be a central part of Microsoft's main thrust in the mobile phone market, which is centered on the revamp of the Windows software it licenses to handset makers, which will be available later this year.

Microsoft hopes its Windows Phone 7, launched with great fanfare in February, will win back share from BlackBerry maker Research in Motion Ltd and iPhone maker Apple Inc, and beat back newcomer Google Inc, which is making ground with Android-powered phones and its own Nexus One.

Microsoft is losing share fast in the U.S. smartphone market, according to tech research firm comScore, dropping 4 percentage points to 15.1 percent between November and February.

Ahead of it are Apple, with 25.4 percent, and Research in Motion with 42.1 percent. Google is the fastest-growing rival, now holding 9 percent of the market.

(Reporting by Bill Rigby and Sinead Carew; Editing by Bernard Orr)

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