* Euro zone finance ministers begin third attempt at Greek deal
* Investors to take profits after last week's strong finish
* Qatar cashes in its warrants in Barclays, causing shares to dip
* Knight Capital in talks for unit sale; shares jump
* Indexes: Dow off 0.5 pct; S&P off 0.4 pct; Nasdaq flat
By Angela Moon
NEW YORK, Nov 26 (Reuters) - U.S. stocks fell on Monday as investors returned to the market after a holiday, focused on euro zone talks to release aid to Greece and negotiations in Washington to avoid the U.S. "fiscal cliff."
Investors were also ready to book profits after major indexes ended last week with gains of 3 percent to 4 percent. The Dow and S&P 500 both closed above key technical levels for the first time since Nov. 6, which could provide additional support. The Dow ended above 13,000, while the S&P broke above 1,400.
"I think the concerns (regarding talks on Greece and U.S. fiscal cliff) are just an excuse for the market to book profits after Friday's surge. We are seeing technical adjustments today," said Peter Cardillo, chief market economist at Rockwell Global Capital in New York.
Euro zone finance ministers and the International Monetary Fund will seek to unfreeze the second bailout package for Greece on Monday, but they first need to agree if some of the official loans to Athens might eventually be forgiven to cut Greek debt.
U.S. lawmakers have made little progress in the past 10 days toward a compromise to avoid the harsh tax increases and government spending cuts scheduled to start taking effect on Jan. 1, a senior Democratic senator said on Sunday.
The Dow Jones industrial average was down 72.03 points, or 0.55 percent, at 12,937.65. The Standard & Poor's 500 Index was down 6.47 points, or 0.46 percent, at 1,402.68. The Nasdaq Composite Index was down 1.11 points, or 0.04 percent, at 2,965.75.
Stocks markets were closed on Thursday for the Thanksgiving holiday and the trading session was shortened on Friday.
Knight Capital Group Inc is in talks about possibly selling its market-making operation, its largest and most profitable business, but it is not known if a deal will happen, sources familiar with the matter said on Saturday. The stock jumped 10 percent to $2.74.
Qatar has cashed in its remaining warrants in Britain's Barclays Plc, a move that should yield a $280 million profit. The warrants have not yet been converted, but conversion would dilute the holding of shares by other investors. U.S.-listed shares of Barclays fell 5.3 percent to $15.41.
Apple Inc has asked a federal court to add six more products to its patent infringement lawsuit against Samsung Electronics Co, including the Samsung Galaxy Note II, in the latest move in an ongoing legal war between the two companies. Apple shares were up 1.2 perecent at $578.17.
U.S. shoppers went to stores earlier this Thanksgiving weekend and bought online more than in years past, giving retailers a strong start to the holiday shopping season, data showed on Sunday. Total spending for the long weekend rose to $59.1 billion, up 12.8 percent from last year, according to the National Retail Federation.
Black Friday's online sales topped $1 billion for the first time ever as more consumers used the Internet do their early holiday shopping, comScore Inc said on Sunday.