“There can be no doubt about it, the big elephant in the room continues to be trade policy,” said Torsten Slok, chief international economist at Deutsche Bank.
Companies are already paying higher prices for materials such as steel and soybeans and are beginning to feel uncertain about how to plan for the future, Mr. Slok said. “This is hanging as a big dark cloud over the spring meetings this year,” he said.
Mr. Trump’s “America First” approach to economic policy has sometimes brought him into conflict with international organizations, which he has criticized as ineffectual. The I.M.F. cautioned Tuesday that the multilateral, rules-based trading system of the World Trade Organization was “in danger of being torn apart.”
Another risk: Mr. Trump’s tax cut. Along with other budget changes, it is expected to boost the United States economy by 1.2 percent by 2020, according to the I.M.F.
But the risk is that the tax cuts cause the economy to overheat, spurring inflation. That could provoke central banks to raise interest rates, which could derail future growth and cause financial problems in fast-growing but risky emerging markets.
In the short run, though, the I.M.F. expects the United States economy to grow by 2.9 percent in 2018 and 2.7 percent in 2019 — up from 2.3 percent in 2017 and faster than the fund’s previous projections.
Speaking at a rally in Hialeah, Fla., on Monday, Mr. Trump cheered the effects of the tax cut and the strength of the economy. “This could be one of the greatest booms ever,” the president said. “Congratulations, enjoy your money.”
Outside the United States, global growth is roaring along, lifted by surging exports in China and strong consumption in India. Prices of commodities are going up after two years of declines, and that is good news for countries, like Brazil and Russia, that are big exporters of oil, gas, metal and agricultural products.
As it did in earlier reports, the I.M.F. emphasized that the prolonged period of economic growth would eventually end and that policymakers needed to act now prepare for the next downturn. The fund urged governments to make tough adjustments to increase their economic productivity now, while growth remained strong.