Shares in Australian ports and rail operator Asciano Group soared Monday after the company received an unsolicited 2.89 billion Australian dollar ($2.69 billion) takeover offer from a U.S. private equity group and a fund.

TPG Capital and independent investment fund Global Infrastructure Partners bid to buy all of Asciano for A$4.40 ($4.10) cash a share, Asciano said in a statement to the Australian Securities Exchange. There was also a share-exchange alternative, Asciano said.

Asciano said in a second statement it would not allow the private equity group and the fund to perform due diligence.

“The Directors have considered the proposal and believe it undervalues the business,” it said in a statement to the Australia Securities Exchange.

Asciano securities rose 16.39 percent to close at A$4.83 ($4.50) on Monday after coming out of a trading halt. The shares closed at A$4.15 on Friday.

A representative for TPG and GIP _ a fund formed by Swiss bank Credit Suisse Group and General Electric Co. _ said no further details could be provided at this stage.

Austock Securities analyst Andrew Chambers said the indicative bid was too low.

“In our opinion, the indicative proposal of A$4.40 (a share) is a low-balled bid that significantly undervalues Asciano,” Chambers said in a briefing note to clients.

He said the price of Asciano securities had been trading at low values because of concerns about the company’s debt load, downgraded earnings guidance and now-abandoned stake in pallet provider Brambles.

He said there was also uncertainty over how Asciano would fund its growth.

“Given all the noise around Asciano, many market participants have largely forgotten the strategic positioning and value of Asciano’s asset base and hence allowed the stock to trade down to levels well below fair value,” Chambers said.

Asciano is due to report its results for the quarter ended June 30 later this week.