* Futures off: Dow 52 pts, S&P 5.25 pts, Nasdaq 10 pts
By Chuck Mikolajczak
NEW YORK, Sept 23 (Reuters) - U.S. stocks were set for a lower open on Tuesday, putting the S&P on pace for a third straight fall, as conflict in the Middle East intensified and after the U.S. Treasury moved to curb "tax inversion" deals.
The United States and Arab allies bombed Syria for the first time on Tuesday, killing dozens of Islamic State fighters and members of a separate al Qaeda-linked group, pursuing a campaign against militants into a war at the heart of the Middle East.
Shares of AbbVie were down 4.9 percent at $55.85 in premarket trading after the Obama administration took several actions late Monday that will reduce the tax benefits available to companies that have done inversion deals, while also making new inversions more difficult and potentially less rewarding.
AbbVie has agreed to a deal to acquire Shire, which fell 4 percent $246.05.
"The market has been looking for an incontrovertible reason to sell-off, to reset before earnings season and we are getting plenty of that," said Peter Kenny, chief market strategist of Clearpool Group in New York.
"In and of themselves, each one of the themes that is driving fear into the market is manageable, combined and as a collective they force the market to move lower."
S&P 500 e-mini futures were falling 5.25 points and fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract, indicated a lower open. Dow Jones industrial average e-mini futures fell 52 points and Nasdaq 100 e-mini futures lost 10 points.
Data due on Tuesday includes the flash September reading on manufacturing from financial data firm Markit at 9:45 a.m. (1345 GMT). Expectations call for a reading in the main purchasing managers' index of 58 versus the final 57.9 in August.
Salix Pharmaceuticals was trading up 7 percent at $171.06 in the premarket after a person familiar with the matter said on Monday that Allergan Inc had revived discussions to buy the company.
Norway's Yara and Chicago-based CF Industries are in talks about a merger of equals that could create a $27 billion global fertilizer producer, rivaling Canada's Potash Corp in size in a fragmented and oversupplied market. Shares in CF Industries were trading up 5 percent to $268.50 in premarket.
CarMax shares slumped 6.3 percent to $49.50 before the opening bell after the largest U.S. used-car retailer reported a lower-than-expected quarterly profit.
Reporting by Chuck Mikolajczak; Editing by Lisa Von Ahn and Nick Zieminski