A Chinese microchip is seen through a microscope set up at the booth for the state-controlled Tsinghua Unigroup project which is aimed at driving China’s semiconductor ambitions during the 21st China Beijing International High-tech Expo in Beijing, China, on May 17, 2018. China has imposed export curbs on two metals used in computer chips and solar cells, expanding a squabble with Washington over high-tech trade ahead of Treasury Secretary Janet Yellen’s visit to Beijing this week. Ng Han Guan/Associated Press

BEIJING — China has imposed export curbs on two metals used in computer chips and solar cells, expanding a squabble with Washington over high-tech trade ahead of Treasury Secretary Janet Yellen’s visit to Beijing this week.

The controls on gallium and germanium are intended to “safeguard national security,” the Commerce Ministry said late Monday. It said exports will require official permission once the rules take effect Aug. 1 but did not say what restrictions might be applied.

Chinese leader Xi Jinping’s government is frustrated by U.S. curbs on access to advanced processor chips and other technology on security grounds. But Beijing has been slow to retaliate, possibly to avoid disrupting China’s fledgling developers of chips, artificial intelligence and other technology.

Yellen is due to arrive Thursday as part of efforts by the Biden administration to revive relations that have plunged to their lowest level in decades due to disputes about technology, China’s military buildup, human rights and other irritants.

China is the biggest global source of gallium and germanium, which are produced in small amounts but are needed to make computer chips for mobile phones, cars and other products, as well as solar panels and military technology.

The United States gets about half its supply of both metals directly from China, according to the U.S. Geological Survey. China exported about 23 metric tons (25 tons) of gallium last year and produces about 600 metric tons (660 tons) of germanium per year.

The United States has blocked Chinese access to advanced chips and the technology to manufacture them and has persuaded allies the Netherlands and Japan to limit exports of chipmaking tools.

That threatens to delay or derail the ruling Communist Party’s ambitions to make China more prosperous and increase its global influence by becoming a leader in clean energy, telecoms, artificial intelligence and other technologies.

In May, Beijing banned use of products from Micron Inc., the biggest U.S. producer of memory chips, in computers and network equipment deemed sensitive.