New York - US utility owner Exelon Corp's bid to acquire power producer NRG Energy Inc turned bitter Wednesday, with Exelon taking its 5.71-billion-dollar offer directly to shareholders after being rejected by the NRG management.

The acquisition would make Exelon the largest power producer in the United States. NRG is said to have rejected the bid partly because it believes that Exelon couldn't guarantee financing.

NRG chief executive David Crane said Tuesday that Exelon was undervaluing his company, according to Bloomberg financial news agency.

Chicago-based Exelon offered NRG shareholders 0.485 of a share of Exelon for each of the 233 million outstanding shares, making the deal worth 5.71 billion dollars based on Exelon's closing price on Tuesday. It had made the same offer to NRG's management last month.

NRG said an exchange ratio of 0.67 Exelon share for NRG would be a fair deal.

"While we would have greatly preferred to enter into direct negotiations with NRG's board and management, their decision to reject our proposal, has left us no choice but to bring the offer directly to the NRG shareholders," Exelon chief executive John W Rowe said. (dpa)