China’s exports of two rare minerals essential for manufacturing semiconductors fell to zero in August, a month after Beijing imposed curbs on sales overseas, citing national security.

China produces about 80% of the world’s gallium and about 60% of germanium, according to the Critical Raw Materials Alliance, but it didn’t sell any of the elements on international markets last month, Chinese customs data released on Wednesday showed. In July, the country exported 5.15 metric tons of forged gallium products and 8.1 metric tons of forged germanium products.

When asked about the lack of exports last month, He Yadong, a spokesperson from China’s commerce ministry told a press briefing Thursday that the department had received applications from companies to export the two materials. Some applications had been approved, he said, without elaborating.

The curbs are indicative of China’s apparent willingness to retaliate against US export controls, despite concerns about economic growth, as a tech war simmers.

Analysts say restricting exports is a “double-edged sword” that may hurt the Chinese economy and accelerate the shift of supply chains out of the country.

China may be the industry leader in producing the two elements, but there are alternative producers, as well as available substitutes for both materials, Eurasia Group analysts said in a July research report.