* Investors fret over Argentina default fallout
* Exxon, Whole Foods both fall after results
* Jobless claims rise more than expected in latest week
* Futures down: Dow 132 pts, S&P 16.5 pts, Nasdaq 35.25 pts (Updates prices, adds jobless claims data)
By Ryan Vlastelica
NEW YORK, July 31 (Reuters) - U.S. stock index futures pointed to a sharply lower Wall Street open Thursday, with the Dow and S&P 500 likely to turn negative for the month on concerns over the strength of overseas economies and ongoing tensions with Russia.
* Data showed U.S. jobless claims rose more than expected in the latest week, a discouraging sign on the labor market ahead of the closely watched July payroll report on Friday.
* Portugal's Banco Espirito Santo's slumped almost 40 percent to an all-time low as the bank's hopes of raising capital without taking state aid suffered a major blow following massive losses.
* In Russia, Moscow fought back over new U.S. and EU sanctions that had been imposed over Ukraine, announcing a ban on most fruit and vegetable imports from Poland.
* Separately, Argentina defaulted for the second time in 12 years. Investors had hoped for a midnight deal with holdout creditors in Argentina, but the plan fell through. Even a short default will raise companies' borrowing costs, add to pressure on the peso, drain dwindling foreign reserves and fuel one of the world's highest inflation rates.
* "Even though the default wasn't terribly unexpected, it creates a concern of systemic risk," said James Dailey, portfolio manager of TEAM Asset Strategy Fund in Harrisburg, Pennsylvania. "On top of that, we haven't had any real downside for a while, and anyone who tries to argue that the market isn't at the very upper end of its historical range is flawed."
* Argentinian stocks traded in the U.S. were lower in premarket, including YPS SA, down 5.3 percent to $36.85 and Pampa Energy, down 5 percent to $10.75.
* The drop in futures suggested Wall Street could have one of its biggest daily declines since July 17, when the S&P 500 fell more than 1 percent after a Malaysian passenger jet was shot down in Ukraine, stoking concerns that the crisis would have a wider global reach.
* S&P 500 e-mini futures fell 16.5 points and were below fair value - a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract - suggesting a lower open. Dow Jones industrial average e-mini futures fell 132 points and Nasdaq 100 e-mini futures lost 35.25 points.
* If the losses hold, it would be enough to erase the monthly gains for the Dow and S&P, which are currently up 0.3 percent and 0.5 percent in July, respectively. The Nasdaq is up 1.2 percent and remains on track for its third straight positive month.
* Dow component Exxon Mobil Corp posted second-quarter earnings that beat expectations, but shares fell 1.5 percent to $101.70 before the bell.
* Late Wednesday, Whole Foods Markets Inc cut its 2014 forecasts for a fourth time, sending shares down 5.4 percent to $37 in premarket trading.
* Cigna Corp reported second-quarter earnings that beat expectations while Time Warner Cable Inc posted a rise in both earnings and revenue. Cigna fell 2.7 percent to $91 before the bell.
* Akamai Technologies declined 6.3 percent to $56.90 in premarket trading a day after it fell short of investor hopes it would exceed its own revenue forecast due to heavy World Cup traffic. (Editing by Bernadette Baum)