President Donald Trump announced the shock 25 percent tariffs on steel and 10 percent duty on aluminium in March, sparking a global diplomatic tussle that sent the markets into choppy waters for weeks.
The tariffs are due to enter into force from June 1, despite efforts by world leaders including France's Emmanuel Macron and Germany's Angela Merkel to bring the multilateral trade system back from the brink.
Separately, the United States is readying trade sanctions against China over intellectual property theft.
"First and foremost, an escalation in trade tensions should be avoided," the Organisation for Economic Cooperation and Development's acting chief economist Alvaro Pereira said.
"Since the world economy is much more integrated and linked today than in the past, a further escalation of trade tensions might significantly affect the economic expansion and disrupt vital global value chains," he added.
Threat to growth
The OECD's last-ditch appeal follows similar warnings from the World Trade Organization and the International Monetary Fund.
The protectionist threat from Trump's "America First" administration is looming large over the outlook for global growth, which the OECD has put at 3.8 percent, close to the pre-financial crisis average.
The organisation meanwhile voiced concern over the recent spike in oil prices.
The rise in prices follows a deal reached in 2016 between the OPEC cartel and Russia to cut production following a glut that had sent prices crashing and the crude sector into disarray.
The OECD's warning came as the organisation held its annual forum in Paris under the theme of "What brings us together".
The 35-nation OECD was founded in 1961 and membership of the body immediately raises a country's economic profile and opens the possibility for deeper international ties on economic, educational and social matters.
The forum convened as global markets felt the heat from Italy, where a political crisis following the electoral win of an anti-establishment, far-right alliance has raised fears of a fresh eurozone crisis.
Without making any mention of the political turmoil, the OECD said it expected the Italian economy to grow 1.4 percent in 2018, and 1.1 percent in 2019.
US economic growth was expected to stand at 2.9 percent this year and 2.8 percent
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