TORONTO (CP) — The credit rating of Manulife Financial has been placed under the microscope by a U.S. agency over concerns about the insurance giant’s $6.4-billion takeover bid for Canada Life Financial. Among the issues for New Jersey-based A.M. Best Co. are that the unsolicited bid means Manulife has not had a chance to comb through Canada Life’s books, plus Canada Life’s weaker credit rating and the fact it operates in European markets where Manulife is absent. ‘‘The thing that makes this complex is Canada Life is such a diverse international player in areas where Manulife is not,’’ said analyst Robert Adams, noting Canada Life’s European operations could create integration challenges. ‘‘On the Canadian side, Manulife knows Canada Life’s operations quite well and we’re confident there.’’ Manulife’s rating of A++ (Superior) is being reviewed with negative implications, which could lead to a rating downgrade that would increase Manulife’s cost of borrowing. The move comes a day after ratings agency Standard & Poor’s Corp. put Canada Life on credit watch, also with negative implications, calling the Manulife offer of $40 a share a “significant distraction’’ for senior management. The Manulife offer sets the stage for a bidding war over Canada Life, with the list of potential suitors including Winnipeg-based Great West Lifeco. Canada Life said Monday it would not comment on the Manulife offer — already rejected as too low by chief executive David Nield — until after a full meeting of its board of directors is held Thursday or Friday. Canada Life has hired investment bankers BMO Nesbitt Burns and Credit Suisse First Boston to review its options. Canada Life may have had an eye to an uncertain future a month ago when it lured a top executive who helped stick-handle rival Clarica Life Insurance through its merger with Sun Life Financial Services earlier this year. J. David Williamson — formerly executive vice-president and chief financial officer at Clarica — was named senior vice-president of strategic planning and business development at Canada Life on Nov. 7. Industry analysts were quick to question Manulife’s strategic rationale in the bid for Canada Life, wondering why Manulife would want back into the European operations it sold to Canada Life six years ago. ‘‘I’m scratching my head,’’ said analyst Colin Devine of Salomon Smith Barney in New York. Devine made the comment to Manulife chief executive Dominic D’Alessandro during a conference call earlier this week.