Uncertainty surrounding US trade policy is weighing on the world's biggest economy, the Federal Reserve said on Wednesday.

President Donald Trump's on-again, off-again tariffs on key US trading partners have upended global markets and spooked investors.

"The outlook in several districts worsened considerably as economic uncertainty, particularly surrounding tariffs, rose," said the Fed's Beige Book survey of economic conditions.

It added that "uncertainty around international trade policy was pervasive across reports" from the Fed's 12 districts.

Since returning to the presidency this year, Trump has imposed broad additional 10 percent tariffs on most US trading partners. Goods from China in particular, face a staggeringly high added rate of 145 percent.

China has struck back with 125 percent levies on imports from the United States, in what US Treasury Secretary Scott Bessent has described as a trade embargo on both sides.

Information for the Fed's report was collected on or before April 14, after many of Trump's sweeping tariffs had been imposed and began rippling through the country.

Besides levies targeting different countries, Trump has also imposed sector-specific duties on imports of steel, aluminium and automobiles.

Most districts saw "moderate to robust" vehicle sales in recent weeks as buyers rushed to make purchases ahead of tariff-related price increases, the Fed said.

But it noted: "Cuts to federal grants and subsidies, along with declines in philanthropic donations, caused gaps in services provided by many community organisations."

Leisure and business travel were down too, and several districts noted declines in international visitors, the Beige Book said.

While there was mixed performance in the manufacturing sector, two-thirds of districts said activity was little changed or had declined, the report added. (AFP)