HAMILTON, Bermuda—The battle to take over Transatlantic Holdings Inc. heated up further on Wednesday with Validus Holdings Ltd. seeking to replace Transatlantic's board.

In a statement, Validus said the Transatlantic board had acted to “thwart” its unsolicited offer to buy the New York-based reinsurer and that it was filing papers with the Securities and Exchange Commission to replace the Transatlantic board with three independent directors.

The move comes three months after Transatlantic agreed to a friendly merger with Zug, Switzerland-based reinsurer and insurer Allied World Assurance Co. Holdings A.G.

Validus, and later Berkshire Hathaway Inc., sought to upset that deal with their own unsolicited bids for Transatlantic.

Validus alleges that Transatlanic's board set up “barriers designed to prevent Transatlantic from entering into good-faith discussions” about its “superior” offer, including adopting a “poison pill” strategy and entering litigation against Validus.

Transatlantic had filed a plan with the SEC in July that would dilute the holdings of anyone who acquires more than 10% ownership. It also filed a federal lawsuit alleging that Validus made false and misleading statements in bid documents.

Transatlantic also amended corporate bylaws to give the Transatlantic board more control over stockholder meetings, Validus said.

In the filing, Hamilton, Bermuda-based Validus nominated three directors to replace the Transatlantic board: Raymond C. Groth, an adjunct professor of business administration at The Fuqua School of Business at Duke University in Durham, N.C.; Paul G. Haggis, the chairman of the Alberta, Canada-based private equity fund of funds manager Alberta Enterprise Corp.; and Thomas C. Wajnert, a senior managing director of Reno, Nev.-based financial services advisory firm The Alta Group L.L.C.

Validus also said advisory firms Glass, Lewis & Co. and Egan-Jones Proxy Services have issued reports recommending that Transatlantic stockholders vote against Allied World’s proposal when they meet on Sept. 20. Another advisory firm, Institutional Shareholder Services Inc., recently made a similar recommendation, citing the emergence of competing bids for Transatlantic.

After Allied World and Validus announced their offers for Transatlantic, Omaha, Neb.-based Berkshire Hathaway Inc.’s National Indemnity Co. unit offered to buy Transatlantic. All three deals were valued at more than $3 billion when they were announced, but have fluctuated with stock market turbulence.