U.S. Treasury Secretary Steven Mnuchin said July 29 that he foresees faster second-quarter expansion in the U.S. economy continuing in the next few years, Reuters reported.

"I don't think this is a one- or two-year phenomenon. I think we definitely are in a period of four or five years of sustained [3%] growth at least," the newswire cited Mnuchin as saying on the "Fox News Sunday" talk show.

The U.S. economy grew at an annualized rate of 4.1% in the second quarter, its fastest pace since 2014 and higher than the revised 2.2% expansion in the first quarter, driven by strong exports as well as consumer and government spending, according to a first estimate from the Commerce Department's Bureau of Economic Analysis on July 27.

But economists expect the pace to moderate, with forecasts showing growth will tail off to 1.8% by 2020 from around 3% this year, said Bloomberg News.

The International Monetary Fund forecasts U.S. growth of 2.9% in 2018 and 2.7% in 2019.

Société Générale's Stephen Gallagher said U.S. GDP growth is "more likely to undershoot than overshoot 3.0% for all of 2018," expecting the second half to average 2.6% as the addition from trade fades.

"The bigger question revolves around whether this is as good as it gets, or whether we can sustain a 3% plus level of sustained economic expansion," said Michael Hewson, chief market analyst at CMC Markets UK, in a July 30 note.

"If, as President Trump boasts, this is just the start then we can probably expect to see a much faster pace of tightening from the Federal Reserve over the next 12 to 18 months."