US STOCKS-S&P 500 posts worst day since April; indexes down for month

* Fears over earlier Fed rate hike spur selling

* Investors fret over Argentina default fallout, Russia tensions

* Exxon falls after results

* Labor costs post largest gain in more than 5-1/2 years

* Dow down 1.9 pct, S&P 2 pct, Nasdaq 2.1 pct (Updates to close)

By Caroline Valetkevitch

NEW YORK, July 31 (Reuters) - The S&P 500 posted its worst daily decline since April and first monthly drop since January on Thursday as economic data sparked concern that the Federal Reserve could raise interest rates sooner than some have expected.

Data showing that U.S. labor costs recorded their biggest gain in more than 5-1/2 years in the second quarter came a day after the Fed upgraded its assessment of the U.S. economy while reiterating it was in no hurry to raise rates.

Problems in overseas economies added to the bearish tone, with Argentina defaulting on its debt for the second time in 12 years.

"If we start to see wages moving higher that's got a be a signal that we are getting closer to the Fed's mandates," said Quincy Krosby, market strategist at Prudential Financial (NYSE: PJH - news) in Newark, New Jersey.

Policymakers for the Fed, which has kept overnight rates near zero since December 2008, at the close of a two-date meeting on Wednesday took note of both faster economic growth and a decline in the unemployment rate, but expressed concern about remaining slack in the labor market.

On Thursday, all 10 macro S&P 500 sectors ended down on the day, with energy down 2.4 percent and leading the decline. Exxon Mobil Corp's second-quarter earnings beat expectations, but its shares fell 4.2 percent to $98.94.

The CBOE Volatility index jumped to its highest level since April.

Based on the latest available data, the Dow Jones industrial average fell 316.8 points, or 1.88 percent, to 16,563.56, the S&P 500 lost 39.39 points, or 2 percent, to 1,930.68, and the Nasdaq Composite dropped 93.13 points, or 2.09 percent, to 4,369.77.

The S&P 500 and Nasdaq both marked their biggest daily percentage drops since April 10. For the Dow, the percentage drop was the biggest since Feb. 3.

In a signal of possible further losses ahead, the S&P 500 closed below its 50-day moving average for the first time since April 15.

For the month, the Dow was down 1.6 percent, the S&P 500 was down 1.5 percent, and the Nasdaq was down 0.9 percent.

Investors on Wednesday had hoped for a midnight deal over Argentina's debt talks with holdout creditors, 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. (Additional reporting by Rodrigo Campos; Editing by Leslie Adler)