* 'Insurmountable antitrust problems' for NYSE-Nasdaq deal
* NYSE counterbid for Nasdaq not being considered
* Board to weigh value, risks of Nasdaq's unsolicited bid
* Nasdaq shares rise 3.7 percent; NYSE up 1.2 percent (Adds antitrust lawyer, updates shares)
By Paritosh Bansal and Jonathan Spicer
NEW YORK, April 6 (Reuters) - A merger between NYSE Euronext and Nasdaq OMX would be strategically unattractive, and any bid would face "insurmountable antitrust problems," a source close to the Big Board said on Wednesday.
A counterbid by NYSE for Nasdaq is "absolutely not under consideration," the source close to the NYSE said, knocking down a media report earlier this week that the exchange operator was considering such a move.
A spokesman for the New York Stock Exchange parent company backed that up, saying on Wednesday: "NYSE is not considering a counterbid for Nasdaq." He did not comment on how NYSE views Nasdaq's unsolicited bid.
The focus on antitrust problems suggests NYSE's board, which is set to meet within a week, could look beyond the premium Nasdaq said it would pay for the Big Board and possibly reject Nasdaq's bid outright.
On Friday, Nasdaq and IntercontinentalExchange unveiled a bid to buy NYSE Euronext for about $11.3 billion in cash and stock -- an attempt to top a $10.2-billion offer made by German competitor Deutsche Boerse.
Such a deal, if it could be completed, would lead to massive job losses in New York City and destroy value for NYSE Euronext shareholders, said the source, who requested anonymity because the talks are not public.
Combining Nasdaq and the NYSE would bring the top two U.S. stock exchanges together with a virtual monopoly on listings, and dominance in trading U.S. cash equities and options. Deutsche Boerse's plan would create the world's top financial exchange reliant on the more profitable derivatives trading.
NYSE's board will review Nasdaq's unsolicited offer based on its value and any risks that it will not ultimately close, including the threat regulators would block it.
"'Insurmountable' is probably an overstatement. But there is substantial antitrust risk," said Jonathan Grossman, an antitrust lawyer at Cozen O'Connor in Washington, D.C.
"If the price difference doesn't justify the risk of the U.S. antitrust regulators objecting to the transaction, then it may very well be a rational decision on their part to reject the Nasdaq/ICE offer," he said of the board members.
The board plans to meet before April 14, but no firm date had been set as of Tuesday, a separate source said.
Nasdaq shares were up 3.7 percent at $29.05, while NYSE rose 1.2 percent to $39.45. (Reporting by Paritosh Bansal and Jonathan Spicer, editing by Lisa Von Ahn and Maureen Bavdek)