Sandell Asset Management Corp., the activist hedge-fund firm run by Tom Sandell, said PartnerRe Ltd. directors hurt shareholders by pushing a merger with AXIS Capital Holdings Ltd. rather than talks with a hostile bidder.

“We will not hesitate to exercise the rights available to us to hold the board accountable,” Tom Sandell said in a letter Friday to Jean-Paul Montupet, the chairman of Bermuda-based PartnerRe.

PartnerRe announced Friday that it would ask shareholders to approve a combination with AXIS, days after saying it was prepared to enter discussions with EXOR SpA over a $6.8 billion bid. EXOR refused Thursday to raise the offer, and PartnerRe said constructive talks were impossible because the Turin-based company insisted that its plan be deemed “reasonably likely to be a superior proposal.”

Sandell has a stake of less than 1 percent of PartnerRe, Dan Zacchei, a spokesperson for the activist firm at Sloane & Co., said by phone. The fund has bought shares in companies including restaurant chain Bob Evans Farms Inc. and pipeline operator SemGroup Corp. and pushed for board changes and spinoffs.

The hedge-fund firm said that the “superior proposal” designation sought by EXOR wouldn’t jeopardize a possible combination with AXIS.

“Rather, the board would be free to negotiate with EXOR before determining, in good faith, whether to change its recommendation, while simultaneously initiating a process that would permit AXIS to improve its offer,” Sandell wrote. “We would once again like to remind the board that its first and foremost duty is to the company’s shareholders.”

PartnerRe pared losses after Sandell’s letter, and traded at $133.03 as of 4:02 p.m. in New York. That compares with Thursday’s close of $133.53 and Turin-based EXOR’s offer of $137.50 per share.

EXOR, the investment firm led by Italy’s billionaire Agnelli family, accumulated a stake of more than 9 percent in PartnerRe and had prepared proxy materials to allow investors to vote against the AXIS merger. EXOR urged PartnerRe in a statement Friday to set a date for a shareholder meeting without delay.

(With assistance from Beth Jinks in San Francisco.)