(Updates, adds CME declines to comment)

NEW YORK, April 16 (Reuters) - A group of NYMEX Holdings Inc NMX.N shareholders are threatening to oppose its merger with CME Group Inc CME.N if CME does not raise its offer price or attempt to boost the value of its stock.

The shareholder group gave a petition, signed by 10 percent of the membership, to the exchange earlier this week, adding the move will make it hold a special meeting on June 3, according to an earlier report in the New York Post.

CME offered 0.1323 shares of its class A common stock and $36 in cash for each share of NYMEX under its offer for the energy and precious metals mart.

But the overall value of the deal has fallen by $2 billion to $9 billion because of the decline in CME's stock price.

Citing sources, the NY Post also reported that, as the deal nears the finish line, CME plans to unveil a stock buyback plan that could prop up its stock price, or as a way to appease the shareholders.

A CME spokesman declined to comment and NYMEX did not return a call seeking comment. (Reporting by Aarthi Sivaraman and Emily Chasan; Editing by David Holmes/Andre Grenon)