(REUTERS) - Dutch delivery firm TNT Express said on Friday its board of directors has rejected an unsolicited takeover offer from U.S-based peer UPS worth an estimated 4.9 billion euros ($6.45 billion), but it said discussions are continuing.

UPS made an offer of 9 euros per share for all of TNT's issued capital, a premium of 42 percent over Friday's closing price of 6.343 euro for TNT shares.

The bid comes as activist shareholders in TNT have stepped up calls for the company to shake up its board and increase shareholder value.

TNT this month agreed to appoint several new members to its supervisory board, including two candidates proposed by American investor Jana Partners, which together with Canada's Alberta Investment Management Corporation (AIMCO,) owns 5 percent.

(Reporting by Roberta B. Cowan and Anthony Deutsch)