China’s cyberspace regulator said Thursday that it had summoned representatives of U.S. tech giant Nvidia to explain alleged security vulnerability risks involving its highly sought-after H20 artificial intelligence chips.

The US lifted its April ban on Nvidia’s H20 chip sales to China earlier this month. Following the reversal, both Nvidia and Advanced Micro Devices Inc. quickly resumed operations in the Chinese market, reclaiming their positions as leading AI chip suppliers to the country.

Twenty national security experts and former government officials urge the Trump administration to reverse a decision to let Nvidia sell H20 AI chips in China.