NEW YORK ( TheStreet) -- Major U.S. stock markets sharply dropped after Secretary of State John Kerry said there was "undeniable" evidence that Syria had used chemical weapons, leaving investors to wonder whether the U.S. will take overt military action to unseat the Assad regime.

The S&P 500 lost 0.4% to 1,656.78. The Dow Jones Industrial Average dropped 0.43% to 14,946.46. The Nasdaq slipped 0.01% to 3,657.57.

"I think any confirmation on use of chemical weapons would obviously be a potential for some kind of action," said Alan Gayle, senior strategist at RidgeWorth Investments. "The market hadn't sold off hard or anything, it's just gone back."

COMEX gold for the December futures contract during electronic trading was jumping $8.90 to $1,402, a critical technical trading level, following comments by Kerry.

Durable goods orders declined sharply by 7.3% in July, the Commerce Department reported Monday before the market open. It was the largest drop since August 2012, snapping three straight months of gains. Economists on average were expecting a decline of 4%.

Amgen ( AMGN) was the top gainer in the S&P, surging 7.7% to $113.75 after the world's largest biotechnology company by sales reached a deal to buy cancer drugmaker Onyx Pharmaceuticals ( ONXX) for about $10.4 billion in cash. Amgen will acquire Onyx for $125 a share; the deal is expected to be completed at the beginning of the fourth quarter. Onyx Pharmaceuticals was jumping more than 5% as well to $123.51.

TMS International ( TMS) surged 12.3% to $17.48 after the provider of procurement and other services to steel mills agreed to sell itself for about $1 billion, including debt, to an investment firm run by Tom Pritzker, a member of one of America's wealthiest business families, people familiar with the matter told The Wall Street Journal.

America Movil SAB de CV ( AMX) tacked on 1.5% to $19.73. Spain's Telefonica SA on Monday increased its agreed offer for E-Plus, the German cellular division of Royal KPN NV, to win the backing of major KPN shareholder America Movil SAB de CV.

But America Movil, Mexican billionaire Carlos Slim's group, which said it now owns 29.77% of KPN, reiterated it still plans to go ahead with its proposed €7.2 billion ($9.65 billion) bid in cash for the remaining shares in the Dutch group, Reuters reported.

Peabody Energy ( BTU ) was another top gainer, advancing 2.2% to $18.25 after Barron's wrote that the coal producer's shares may be poised for upside on stronger coal prices as utilities begin moving back to coal from natural gas.

C.H. Robinson Worldwide ( CHRW ) meanwhile added 2.9% to $58.83 after analysts at Deutsche Bank maintained a "buy" view on shares of the logistics company and said that its announcement to increase its share repurchase initiative by 15 million shares will have a strong impact on the stock price and squeeze shorts.

The benchmark 10-year Treasury was rising 7/32, diluting the yield to 2.789%.

-- Written by Andrea Tse and Joe Deaux in New York

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