About 25 minutes into trade, the Dow Jones Industrial Average was at 17,564.36, down 59.51 points (0.34 percent).
The broad-based S&P 500 shed 7.19 (0.35 percent) at 2,044.41, while the tech-rich Nasdaq Composite Index dropped 8.00 (0.17 percent) to 4,800.87.
Delta Air Lines lost 2.7 and United Continental shed 2.1 percent on news of the deadly explosions at the Brussels airport and a metro train in the Belgian capital.
Also lower were hotel chain Marriott International, down 2.2 percent, and online travel companies Priceline and Expedia, which lost 2.0 percent and 1.7 percent, respectively.
Cruise companies also fell, with Carnival dropping 3.1 percent and Royal Caribbean Cruises 3.4 percent.
Banking shares were lower, with Dow members JPMorgan, Bank of America and Citigroup all down about 0.6 percent.
Macy's dropped 1.0 percent as it named William Lenehan, chief executive of the Four Corners real estate investment trust, to its board of directors. The move comes as Macy's faces pressure from some investors to better monetize its real estate holdings.