Nasdaq OMX Group Inc and IntercontinentalExchange said NYSE Euronext investors should be highly skeptical of the additional merger cost savings outlined by NYSE and Deutsche Boerse AG.

Nasdaq and ICE are hoping to thwart the Deutsche Boerse deal, having offered $11.2 billion to buy NYSE after Deutsche Boerse and NYSE agreed to a $9.8 billion merger.

Although Nasdaq and ICE sweetened their offer twice, NYSE's board and management have rejected the rival bid as overly risky.

The bidding war has raised questions about which deal is better for shareholders, given the price disparity and regulatory hurdles. On Sunday, NYSE and Deutsche Boerse said they had discovered some 400 million euros ($580 million) of cost savings in their deal.

In a statement on Monday, Nasdaq and ICE said that the newfound cost savings were not the result of sharpening a pencil, but an unexplained shift in strategy. The exchange operators also noted that shareholders have not been offered an increase in price.

The deals still need shareholder approval before moving forward. NYSE shareholders will have their annual meeting on April 28. Deutsche Boerse shareholders are expected to vote on July 7.

(Reporting by Lauren Tara LaCapra; Editing by Derek Caney)