Tension is rising between Syria and the West. U.S. Defence Secretary Chuck Hagel said Tuesday that U.S. forces are ready to strike if President Barack Obama gives the order.
The Dow Jones industrial average was down 91 points, or 0.6 per cent, at 14,854 as of noon EDT Tuesday.
The Standard & Poor's 500 index fell 16 points, or 1 per cent, to 1,640. The Nasdaq composite fell 45 points, or 1.3 per cent, to 3,611.
Airline stocks plunged as traders worries about higher fuel costs.
The price of crude oil jumped 2.88, or 2.7 per cent, to $108.79 a barrel, close to its highest level this year.