Washington is "aggressively" warning Beijing over the repercussions of its demand that US tech firms should hand over their encryption keys if they want to do business in China, a top US official said Tuesday.

The statement by US Commerce Secretary Penny Pritzker comes one month after President Obama rebuked Beijing over a new anti-terror law that would require companies to give Beijing details of their encryption methods or be denied access to the Chinese market.

The law, recently drafted by Chinese lawmakers, is expected to be adopted this year.

"The approach that we've taken with the Chinese government is, one, to first of all aggressively talk about the challenges that some of the regulations might do to impede trade," said Pritzker, who is leading a clean energy trade mission to China with executives from 24 US companies.

She added that she had had "pointed conversations" with Chinese leaders about the issue on Monday.

China operates a vast security and surveillance apparatus, with the ruling Communist Party maintaining a resolute grip on power, while Washington and Beijing frequently trade accusations of state-sponsored cyber-spying.

Beijing is tightening its grip on information after a series of deadly attacks which authorities have blamed on separatists from the far-western, mainly Muslim region of Xinjiang.

The foreign ministry in Beijing says China is a victim of hacking and has defended the law as purely internal and "a requirement for the Chinese government to prevent and combat terrorism".

Yet the measure has alarmed some US-based tech companies and has drawn criticism from Obama, who said last month that he had raised the issue directly with Chinese President Xi Jinping.

The row comes amid reports that Beijing has expanded its Internet censorship efforts beyond its borders with a new strategy, dubbed the "Great Cannon", that attacks websites across the globe.

In a roundtable with Chinese and foreign media, Pritzker said that Washington wants to ensure US firms in China can "keep the promises they've made to their customers".

"What we talked about is having a dialogue -- a dialogue being where two governments come together to really seriously have a conversation about how to address these issues," she said. "And there seemed to be some receptivity to doing that."