BERLIN — President Donald Trump’s trade war is expected to slow growth in the world’s leading economies, including the United States, this year and in the years to come, unless world leaders can resolve their differences over trade.

The Organization for Economic Cooperation and Development slashed its outlook for global output to 2.9% this year, from 3.3% in 2024, the organization said in its economic report released Tuesday.

Economic growth in the United States is expected to be particularly weak, the organization said, rising 1.6% this year, a drop from the 2.2% projected in March, and 1.5% in 2026, down from its previous estimate of 1.6%. The U.S. economy grew 2.8% in 2024.

“Through to the end of 2024, the global economy showed real resilience,” said Mathias Cormann, the organization’s secretary-general. “But the global economic environment has become significantly more challenging since.”

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% 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,” Cormann said.

Since taking office, 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 such as Canada, Mexico and the European Union, as well as longtime rivals such as 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 the United States’ trading partners by just 10%, it would shave 1.6% off of 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.

The economies of the 20 countries using the common euro currency are projected to grow 1% in 2025 and 1.2% in 2026, in line with the OECD forecast from March. China’s economy is expected to see 4.7% growth this year and 4.3% in 2026, down 0.1 percentage points from the organization’s spring projection.

Economists in the organization urged countries to reach agreements on trade and to increase investment to revive economic growth.

“Our key recommendation, to all governments, is to engage with each other to address issues in a global trading system cooperatively,” Cormann said.