Chinese exports of rare earths—17 key minerals at the heart of trade tensions with Washington—rose 23% in May compared to the previous month, customs data showed Monday. May shipments totaled 5,865 tons—still below last year’s level—but the monthly surge lifted year-to-date exports by 2.

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.