The U.S. economy grew at a robust annual rate of 4.2 percent in the second quarter, the best performance in nearly four years, but economists believe growth has slowed in the current quarter, in part because of a drag from trade.

The Commerce Department says the performance of the gross domestic product, the country's total output of goods and services, was unchanged from its estimate last month.

The strong GDP performance has been cited by Trump as proof that his economic program is working.

"We're doing much better than anybody thought possible," Trump said at a Wednesday news conference.

However, a big part of that growth reflected a temporary rush to ship soybeans and other U.S. exports out before penalty tariffs triggered by Trump's get-tough trade policies took effect.