Wall St near flat after two days of gains, energy falls

The S&P 500 energy index is down 0.5 percent following sharp declines in oil prices, with shares of Exxon Mobil down 0.5 percent.

Investors are watching closely news on Ukraine. A Russian convoy carrying food, water and other aid set off for eastern Ukraine, but Kiev and Western governments warned Moscow against any attempt to turn the operation into a military intervention.

The Dow Jones industrial average is rising 6.81 points, or 0.04 percent, to 16,576.79, the S&P 500 is up 1.3 points, or 0.07 percent, to 1,938.22 and the Nasdaq Composite is adding 2.10 points, or 0.05 percent, to 4,403.43.

Shares of Tekmira Pharmaceuticals were down 17 percent to $19.54 a share in active trading Tuesday in what's been a wild ride for the stock of late.

The company is developing an Ebola treatment that had been put on hold by U.S. regulators and then later released for treatment in certain patients. Monday, shares closed at $23.80, but short-sellers are having their way Tuesday.

The largest percentage gainer on the New York Stock Exchange is Fortegra Financial, which is up 40.31 percent, while the largest percentage decliner is Gain Capital, down 10.00 percent.

At the NYSE, most active stocks are Kinder Morgan, down 1.22 percent, and Twitter.

On the Nasdaq, most active stocks include NII Holdings, down to 21 cents a share from 65 cents a share, and Exact Sciences, up 7.9 percent to $18.44.

Advancing issues are outnumbering declining ones on the NYSE by 1,378 to 1,290, for a 1.07-to-1 ratio on the upside; on the Nasdaq, 1,324 issues are falling and 902 advancing for a 1.47-to-1 ratio favoring decliners.

The broad S&P 500 index is posting two new 52-week highs and no new lows; the Nasdaq Composite is recording 16 new highs and 22 new lows.

From earlier in the day:

09:00 - U.S. stock index futures are little changed on Tuesday, with the situation in Ukraine continuing to be a major market driver.

While many investors are betting that the situation between Russia and Ukraine would not escalate dramatically, uncertainty continues to swirl around the region. Ukraine reported that Russia had massed 45,000 troops on its border, while Russia said a convoy of 280 trucks carrying humanitarian aid had set off for the country.

Western governments have warned Russia against using help as a pretext for an invasion, which NATO chief Anders Fogh Rasmussen on Monday warned was a "high probability."

Biotech companies are among the strongest in Tuesday's premarket session. Intercept Pharmaceuticals Inc is soaring 46 percent to $346.50 a day after it said its drug to treat liver disease was effective in mid-stage trials. Exact Sciences is up 5.3 percent to $17.99 before the bell after the Food and Drug Administration approved its noninvasive colorectal cancer screening test.

(Reporting by Ryan Vlastelica, Caroline Valetkevitch)