Chinese exports of rare earths, a group of 17 minerals at the center of a trade dispute with Washington, rose 23% in May from the previous month, according to customs data on Monday.

The US trade war with China has a major sticking point: rare earths minerals. Last month, President Donald Trump said the United States needed Greenland “very badly,” renewing his threat to annex the Danish territory.

China has agreed to fast-track approvals for the shipment of rare earth minerals to the United States and some European Union nations.