President Donald Trump's threat to curtail diesel shipments abroad comes with a risk: the perception that he's reneging on his promise to shower the world with U.S. fuels.

That move would inflame tensions between the U.S. and Europe while jeopardizing American credibility as a trading partner, energy experts and administration allies said. And it could compel nations to seek other suppliers, limiting Trump's ability to wield energy as a negotiating tool in the future, they added.

"This is going to damage our reputation," said an external adviser to the Trump administration, who was granted anonymity to discuss private conversations. "The whole premise of energy dominance was that the United States would be able to supply our allies around the world. Curtailing that is going to raise question marks."

Plans to limit U.S. diesel exports are under discussion at the White House as the fuel's price soars because o...