ECB chief Mario Draghi drew a gloomy picture of the eurozone, saying the outlook was getting "worse and worse" as the bank signaled readiness to provide more stimulus to the economy.

Market hopes have been sustained in recent weeks by the belief the US Federal Reserve will cut interest rates next week.

But about 20 minutes into the day's trading, the benchmark Dow Jones Industrial Average was 0.5 percent lower at 27,139.68.

The broad-based S&P also fell 0.5 percent to 3,005.05 while the Nasdaq lost 0.8 percent to 8,253.91.

The Nasdaq was pressured by plunging shares in Tesla, which fell 14.2 percent after the company reported bigger-than-expected losses in the second quarter.

Among other companies, Southwest Airlines dropped three percent after saying it had abandoned hopes of returning its fleet of Boeing 737 MAX aircraft to service before 2020.

The company also said it would suspend operations at the Newark international airport to save money.

Ford Motor Company declined 6.9 percent a day after reporting steep losses tied to restructuring costs and forecasting disappointing full-year earnings.