U.S. stocks fell for a third straight session on Monday as a previously buoyant market limped into earnings season.

The latest worries about the euro zone sovereign debt crisis also weighed on investors, diverting attention from a flurry of merger activity.

The European Central Bank threw Portugal a temporary lifeline on Monday by buying up its bonds, traders said, as market and peer pressure mounted for Lisbon to seek an international bailout soon.

It's certainly a valid concern, said Joe Saluzzi, co-manager of trading at Themis Trading in Chatham, New Jersey. But they all just go away, you ever notice that? They just throw some money at it, and everything is fine, but eventually it won't be.

Topping the list of deals announced on Monday, Duke Energy Corp agreed to buy Progress Energy Inc
for $13.7 billion in stock. DuPont plans to buy Danisco , a Danish food ingredient firm, for $5.8 billion.

Shares of Progress slid 2 percent to $43.81, and Duke fell 2.1 percent to $17.42. DuPont, a Dow component, fell 3.4 percent to $48.06.

Markets had advanced recently ahead of the upcoming earnings season. Last week the Dow and S&P notched a sixth straight week of gains, while the Nasdaq rose 1.9 percent.

But the benchmark S&P 500 was on track for its third straight session of declines, its first three-day losing streak since late November.

Alcoa Inc is scheduled to report its quarterly results after the market closes, unofficially launching the fourth-quarter earnings season. The stock, a Dow component, edged 0.7 percent lower to $16.53.

The Dow Jones industrial average <.DJI> fell 73.11 points, or 0.63 percent, at 11,601.65. The Standard & Poor's 500 Index <.SPX> lost 6.50 points, or 0.51 percent, at 1,265.00. The Nasdaq Composite Index <.IXIC> shed 16.30 points, or 0.60 percent, at 2,686.87.

Education stocks slipped after Strayer Education Inc said new enrollments at its university fell 20 percent in the winter term, signaling another tough quarter ahead from U.S. for-profit colleges.

Strayer shares plunged 22.2 percent to $119.21. Corinthian Colleges Inc tumbled 12.3 percent to $4.63, and Apollo Group Inc lost 5.8 percent to $35.78.

General Electric Co was a bright spot, up 1.4 percent to $18.68, after UBS raised shares of the largest U.S. conglomerate to buy from neutral and added the stock to its U.S. key calls list.

(Editing by Padraic Cassidy)