The US is blocking visas for certain Chinese graduate students and researchers to prevent them from stealing sensitive research, the acting head of the US department of homeland security, said on Wednesday.

Chad Wolf repeated US charges of unjust business practices and industrial espionage by China, including attempts to steal coronavirus research, and accused it of abusing student visas to exploit American academia.

“We are blocking visas for certain Chinese graduate students and researchers with ties to China’s military fusion strategy to prevent them from stealing and otherwise appropriating sensitive research,” he said in a speech in Washington.

Wolf said the US was also “preventing goods produced from slave labour from entering our markets, demanding that China respect the inherent dignity of each human being,” an apparent reference to alleged abuses of Muslims in China’s Xinjiang region.

Wolf did not give details.

US Customs and Border Protection officials have prepared orders to block imports of cotton and tomato products from Xinjiang over accusations of forced labour, though a formal announcement has been delayed.

Sino-US relations have sunk to historic lows with the world's two biggest economies clashing over issues ranging from trade and human rights to Hong Kong and the coronavirus.

US President Donald Trump, who had touted friendly ties with Chinese President Xi Jinping as he sought to make good on promises to rebalance a massive trade deficit, has made getting tough on China a key part of his campaign for re-election on November 3.

He has accused his Democratic opponent Joe Biden, who leads in most opinion polls, of being soft toward Beijing.