Elon Musk has warned Twitter staff the company may not survive if it cannot find new ways to make money, as top executives continue to quit.

Twitter's new owner has been warned he is not "above the law" by a US privacy regulator, as the company loses high-level leaders responsible for data privacy, cybersecurity and compliance with regulations. 

They include Yoel Roth, Twitter's head of trust and safety who oversaw the social media company's bid to combat hate speech. An executive confirmed Roth's resignation to coworkers on an internal messaging board seen by the Associated Press.

Mr Musk ordered workers to return to the office on Thursday morning local time, with his first "all-hands" meeting that afternoon.

"Sorry that this is my first email to the whole company, but there is no way to sugar-coat the message," he wrote.

"We need roughly half of our revenue to be subscription."

At the meeting Mr Musk said some "exceptional" employees could be exempt from the return-to-work order, but others who did not like it could quit, according to an employee who was there, who spoke on condition of anonymity.

The employee also said the Tesla billionaire appeared to downplay staff fears about how a pared-back Twitter workforce would meet privacy and data security standards, replying that as CEO of Tesla, he knew how it worked.

The memo and staff meeting echoed a live-streamed address the day before where Mr Musk assured advertisers his erratic takeover would not hurt their brands, after several companies paused advertising on the platform.

Rocky rollout for new blue checks

Mr Musk told employees the "priority over the past 10 days" was to develop and launch Twitter's new $US7.99 ($12) monthly subscription which includes the blue check mark, which had previously only applied to verified accounts. 

The project has had a rocky rollout this week with an onslaught of fake accounts impersonating high-profile figures including basketball star LeBron James and the drug company Eli Lilly.

In a second email to employees, Mr Musk said the "absolute top priority" over the coming days is to suspend "bots/trolls/spam" exploiting the verified accounts.

But Twitter now employs far fewer people to help him do that.

An executive last week said Twitter was cutting about half its 7,500 strong workforce.

Mr Musk told employees in the email "remote work is no longer allowed", that the road ahead is "arduous and will require intense work to succeed", and they will need to be in the office at least 40 hours a week.

Twitter's exodus includes the company's chief privacy officer, Damien Kieran, and chief information security officer Lea Kissner, who tweeted on Thursday, "I've made the hard decision to leave Twitter."

Roth's resignation is a "huge loss" for Twitter's reliability and integrity, said former co-worker and friend Emily Horne.

"He's worked incredibly hard under very challenging circumstances, including being personally targeted by some of the most vicious trolls who were active on the platform," said Ms Horne, who oversaw global policy communications at Twitter until 2018.

Cybersecurity expert Alex Stamos, a former Facebook security chief, tweeted on Thursday there is a "serious risk of a breach with drastically reduced staff", which could also put Twitter at odds with a 2011 order from the Federal Trade Commission (FTC) which requires it to respond to serious data security lapses.

US regulator has 'deep concern'

"Twitter made huge strides towards a more rational internal security model and backsliding will put them in trouble with the FTC" and other regulators in the US and Europe, Mr Stamos said.

The FTC said it is "tracking recent developments at Twitter with deep concern". 

"No CEO or company is above the law, and companies must follow our consent decrees," the agency said in a statement.

"Our revised consent order gives us new tools to ensure compliance, and we are prepared to use them."

The FTC would not say whether it was investigating Twitter for potential violations.