The Dow Jones industrial fell 64 points, or 0.4 per cent, to 14,946 Monday.
The Standard & Poor's 500 index fell six points, or 0.4 per cent, to 1,656. The Nasdaq composite was down a fraction of a point at 3,657.
Stocks sagged in the last hour of trading after Kerry stepped up criticism of Syria, calling last week's attack a "moral obscenity."
Stocks started the day higher after a handful of corporate deals were announced.
Amgen surged after the biotech giant said it plans to buy Onyx Pharmaceuticals for $10.4 billion.
Three stocks fell for every two that rose on the New York Stock Exchange. Volume was very low at 2.4 billion shares.