U.S. stocks moved higher on Monday as a bounce back in commodities lifted energy and materials shares, offsetting concerns about European debt.

Stocks seesawed for much of the morning between modest gains and losses after Standard & Poor's downgraded Greece's rating into junk territory on doubts Athens can manage its debt without imposing losses on private bondholders.

The energy and materials sectors were the best performers on the S&P 500 as the Reuters/Jefferies commodities index <.CRB> jumped 1.7 percent.

The iShares Silver Trust exchange-traded fund rose 6 percent. Oil futures jumped more than 4 percent and the S&P energy index <.GSPE> advanced 1.6 percent.

The Greece rating cut weakened the euro against the U.S. dollar, shaving gains off commodities and reducing some risk appetite that had lifted futures.

Europe continues to be a regular cloud on the market, dimming the market's bullish mood, said Christian Magoon, chief executive of Magoon Capital in Lisle, Illinois.

The Dow Jones industrial average <.DJI> advanced 61.15 points, or 0.48 percent, at 12,699.89. The Standard & Poor's 500 Index <.SPX> put on 6.84 points, or 0.51 percent, at 1,347.04. The Nasdaq Composite Index <.IXIC> rose 18.03 points, or 0.64 percent, at 2,845.59.

On the S&P 500, 1,340 and 1,333 are key levels that should provide strong support and entice buyers, according to Ryan Detrick, senior technical strategist at Schaeffer's Investment Research.

Despite last week's losses, the S&P 500 held above important technical levels, with the week's low just below 1,330 and Friday's close above 1,340.

Commodities, stocks and the euro have traded in a similar pattern in recent months as low U.S. interest rates boosted risk appetite.

Citigroup Inc , which in recent months accounted for about 6 percent of total composite volume, fell 2.6 percent to $44.02 after the company's 1-for-10 reverse stock split and pressured the market. The S&P financial sector index <.GSPF> was down 0.4 percent.

In merger news, Hertz Global Holdings Inc fired a new salvo in a 13-month-long battle for Dollar Thrifty Automotive Group Inc , offering nearly $2.1 billion -- almost double what it offered in April last year -- trying to create the No. 2 U.S. car rental group.

Dollar Thrifty was up 13 percent at $78.70, while Hertz Global added 0.3 percent at $16.89.

Among companies reporting quarterly results, Sysco Corp gained 12.7 percent to $32.13 after its profit beat analysts' expectations.

About 70 percent of S&P 500 companies that have reported results have beaten expectations.

(Reporting by Angela Moon; editing by Jeffrey Benkoe)

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