In the first three months of the year, economic growth in the countries monitored by the organization, which is based in Paris, “dropped abruptly” to 0.1 percent from the last three months of 2024, which is “the slowest rate of growth since the peak of the Covid-19 pandemic some five years ago,” Mr. Cormann said.

Since taking office, Mr. Trump has imposed tariffs, then halted them for several weeks, then reinstated some, in the hopes of winning new trade deals from once-close allies like Canada, Mexico and the European Union, as well as longtime rivals like China.

The lack of certainty coming from that on-again, off-again strategy, combined with frequent changes in how high the tariffs will eventually be, has roiled markets and disrupted the flow of goods and services around the world. From January to March, many companies rushed goods to the United States, hoping to avoid the higher tariffs, many of which are now set to take effect in July.

Even if the Trump administration increases tariffs on most of America’s trading partners by just 10 percent, it would shave 1.6 percent off economic growth in the country over two years, the report said. Growth on a global scale would contract nearly a full percentage point in the same period.

Further pressure is coming from the need for leading economies, such as those in the European Union, to increase military spending while also investing in the transition to a green economy, the report said.