President Donald Trump said his Chinese counterpart Xi Jinping had agreed to restart the flow of rare-earth materials, as negotiators from the two nations prepare to resume trade talks on June 9 in London.

But there was a discrepancy in the two sides’ statements after the Trump-Xi call on one of the most contentious issues: rare earths, which are needed to make all kinds of advanced technologies and are produced almost entirely in China.

Earlier this week, China hit back on Trump’s claims the Asian country was in breach of a new trade agreement, struck last month following talks held in Geneva, Switzerland, countering that the U.S. itself was undermining the deal with new sanctions.