Trump's former ambassador to Vietnam highlighted the main reason China may hesitate to strike a trade deal with the U.S.

The remarks from both sides raise concerns that a trade war could erupt again between the two largest economies.

But a U.S. official told Reuters that it appears China was moving slowly on promises to issue export licenses for rare earths minerals. The deal called for China to lift trade countermeasures that restrict its exports of the critical metals needed for U.S. semiconductor, electronics and defense production.