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

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.

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

China’s factory activity contracted in May, according to an official survey released on Saturday, although the decline slowed from April as the country reached a deal with the U.S. to slash President Donald Trump's sky-high tariffs.

China's manufacturing activity shrank for a second month in May, an official survey showed on Saturday, raising expectations for more stimulus to support the economy amid a protracted trade war with the United States.

JPMorgan Chase CEO Jamie Dimon sounded a warning Friday on the fractious US relationship with China — and on “the enemy within.”