The directors of Canadian Hydro Developers said on Thursday they are actively considering other options as they again urged shareholders to reject an unsolicited bid by TransAlta Corp.

On July 20, TransAlta, Canada's biggest pure-play electricity firm, said it would offer C$4.55 a share for Canadian Hydro, implying a deal value of about C$654 million ($611 million).

Canadian Hydro promptly rejected the offer, and on Thursday mailed a circular to its shareholders explaining their reasons.

Meantime, the renewable-power company said it is looking at other options, and said superior proposals may emerge.

We are actively pursuing other alternatives and we are encouraged by the level of interest, said Dennis Erker, chairman of the board of directors of Canadian Hydro, in a statement.

This is not a company to be purchased at a discount.

Among reasons for rejection, the board of Canadian Hydro said the TransAlta offer is inadequate and the timing of the offer is opportunistic.

Furthermore, the unsolicited offer is not a permitted bid under Canadian Hydro's shareholder rights plan and the TransAlta bid is highly conditional, Canadian Hydro said. ($1=$1.07 Canadian) (Reporting by Ka Yan Ng, editing by Gerald E. McCormick)