WeChat is ubiquitous among Chinese speakers worldwide, used by more than a billion people for messaging and digital payments among many other features.

The US company, Calif, describes itself as a team of hackers aiming to "make the internet safer for everybody" by finding flaws in apps and other programmes.

Calif said it had used artificial intelligence to build a cyberattack "worm" able to spread from phone to phone simply by making calls to WeChat contacts, but had not unleashed it.

"The victim does not need to answer the call, or interact with their phone at all," said a Calif blog post published Tuesday about its "WeWorm" project.

"Exploitation takes only seconds, and gives us full control of the WeChat account. We can read and send messages, make calls, and act on the victim's behalf," it said.

It added that when chained with other mobile phone bugs, its "WeWorm" could even take full control of a device.

Chinese tech giant Tencent, which runs WeChat, told AFP Calif had notified it of a "potential security issue".

"We investigated the report and have implemented a fix. The fix was deployed on our servers and is now live for all users -- no app update or any other action is required," Tencent said.

"We have no evidence that the issue was exploited or that any user was affected," its statement said, adding the company was "grateful to the researchers for bringing this to our attention".

The ability of powerful AI systems to find previously unknown cybersecurity vulnerabilities has sparked urgent calls for governments and companies to strengthen digital defences.

Calif said it had used AI to find the security vulnerability in WeChat within two days, and to build the worm within one more week -- work it said would previously take a larger team months.

"We are publishing our findings to raise public awareness," its blog post said.

"AI is putting these capabilities in the hands of less-skilled actors, leaving ordinary users at unprecedented risk."

AI could be on the agenda at an expected meeting this month between President Donald Trump and his Chinese counterpart Xi Jinping at the White House.

"The US and China disagree on plenty, but keeping billions of people safe online shouldn't be one of them," Calif chief Thai Duong wrote on social media.