UPDATE 3-RIM opts for share buyback, stock climbs

* To buy back about 21 mln shares

* Analysts say plan may give comfort, but concerns remain

* Stock climbs 3.2 percent to $59.46 on Nasdaq (Adds comments, background; Changes placeline from OTTAWA)

By Wojtek Dabrowski

TORONTO, Nov 5 (Reuters) - BlackBerry maker Research In Motion Ltd (RIM.TO) (RIMM.O) announced a share buyback worth up to $1.2 billion on Thursday, but analysts said competitive concerns will continue to exert pressure on the company's stock.

RIM's shares climbed 3.2 percent after the Waterloo, company said it would repurchase up to 21 million shares, or 3.6 percent of outstanding common shares on Nasdaq. RIM hasn't bought back any stock in the last year.

"Although the share repurchase should give investors some comfort, we believe headwind concerns will likely remain, putting the onus on the company to continue to disprove market concerns through earnings strength," UBS analysts Phillip Huang and Maynard Um said in a note on Thursday.

RIM said the share repurchase program, which will run up to 12 months or until purchases are completed, may commence on Nov. 9.

The shares trade at close to $60 on the Nasdaq. In late September, they hit a year high of $88.08. They have a year low of $35.05, set in March.