* Oil prices lose much of earlier gains
* Viacom down after Redstones withdraw merger approval
* Dow up 0.08 pct, S&P down 0.23 pct, Nasdaq down 0.72 pct (Updates prices, comment, changes byline)
By Rodrigo Campos
NEW YORK, Dec 12 (Reuters) - The S&P 500 and Nasdaq Composite fell on Monday after six sessions of gains, weighed by tech sector stocks, while a rally in energy shares petered out as crude oil gains withered to less than 2 percent from nearly 6 percent earlier.
Bank shares fell ahead of the Federal Reserve's last meeting for the year, which is set to begin on Tuesday and expected to end on Wednesday with the year's first interest rate increase.
Traders cashed in gains in bank stocks ahead of the meeting, even if a rate hike, which on paper would benefit banks, is all but priced in.
The chances that the U.S. central bank will not raise rates is "close to zero, but it's not zero," said Kim Forrest, senior equity research analyst at Fort Pitt Capital Group in Pittsburgh.
The S&P 500 financial sector fell 1.0 percent following five consecutive weeks of gains.
The benchmark's tech sector fell 0.5 percent after posting its largest weekly advance in a year on Friday.
The healthcare sector was the biggest boost to both the S&P 500 and the Dow industrials.
The Dow Jones industrial average was up 15.86 points, or 0.08 percent, to 19,772.71, the S&P 500 had lost 5.16 points, or 0.23 percent, to 2,254.37 and the Nasdaq Composite had dropped 39.09 points, or 0.72 percent, to 5,405.41.
Both the Dow and the S&P hit intraday record highs earlier in the session.
The industrials sector was down 0.5 percent, dragged down by defense stocks. Lockheed Martin declined 2.7 percent to $252.61, bouncing back from a session low of $245.50 after U.S. President-elect Donald Trump tweeted that the company's F-35 program and costs were "out of control".
Other defense stocks, such as General Dynamics, Raytheon, and Northrop Grumman, were also down.
Viacom fell 9.3 percent to $35.01 after National Amusements withdrew its merger proposal for CBS and Viacom. National Amusements is privately held and controlled by Sumner Redstone and his daughter, Shari Redstone.
Declining issues outnumbered advancing ones on the NYSE by a 2.04-to-1 ratio; on Nasdaq, a 2.34-to-1 ratio favored decliners.
The S&P 500 posted 66 new 52-week highs and one new low; the Nasdaq Composite recorded 223 new highs and 25 new lows. (Reporting by Rodrigo Campos; Editing by Jonathan Oatis)