The United States called for China not to use controls over lucrative rare earth metals as a "weapon," concluding that Beijing likely curbed exports to Japan due to political reasons.
China produces more than 95 percent of rare earth products, which are critical to manufacturing 21st-century goods from iPods to low-emission cars to wind turbines.
"All we're asking for is that they not use rare earth products as a trade weapon," Jon Huntsman, the US ambassador to China, told "The Charlie Rose Show" on US public television in a weekend interview.
Japanese industry said China temporarily cut off exports earlier this year during a flare-up in a territorial row between Asia's two largest economies. China denied that the government imposed restrictions.
"I think in the example of Japan it could be argued that in fact it was used as a trade weapon," Huntsman said.
"And of course the feelings there are quite deep and they go back some years," he said of relations between the two countries.
But Huntsman said that on supply of rare earths to the United States, "I don't think the evidence is clear."
He acknowledged that China had more leeway to curb rare earth shipments than other exports under rules of the World Trade Organization (WTO) and the earlier General Agreement on Tariffs and Trade.
"I think that's being looked at," he said of possible WTO violations. "But it is less clear than maybe some of their other WTO-only cases that they violated."
Chinese Commerce Minister Chen Deming, on a visit to Washington last week, flatly denied any political motivation over rare earth shipments. He said Beijing was restricting mining due to environmental concerns and that Chinese companies were also affected.
A recent Energy Department report urged the United States to ramp up efforts to produce rare earth products at home and secure them overseas, warning that the United States otherwise risks losing out to China in the growing area of clean energy.