As the U.S. prepares to impose tariffs on steel and aluminum imports, a world economic watchdog warned Tuesday that such barriers will hurt economic growth globally.

The Organization for Economic Co-operation and Development, a policy adviser to developed economies, said that while the global economy is improving, "trade protectionism remains a key risk that would negatively affect confidence, investment and jobs."

"Governments should avoid escalation," it said in an update to its forecasts.

The statement was a veiled reference to U.S. President Donald Trump's decision to impose tariffs without resorting to international arbitration.

The OECD expects world economic growth to accelerate to 3.9 percent this year and next, from 3.7 percent in 2017. It expects the U.S. to accelerate thanks to lower taxes, and sees a pick-up also in several developing countries. By contrast, it predicts a gradual slowdown in the 19-country eurozone, Britain, China and Japan.