The future of Twitter seemed to hang in the balance Friday after its offices were locked down and key employees announced their departures in defiance of an ultimatum from new owner Elon Musk.

Fears grew that the fresh exodus would threaten the very existence of one of the world's most influential internet platforms, which serves as a key communication tool for the world's media, politicians, companies and celebrities.

Mr Musk, also the CEO of Tesla and SpaceX, has come under fire for radical changes at the California-based firm, which he bought less than a month ago for $44 billion.

In an internal memo sent this week, Mr Musk had told workers that they must choose to be "extremely hardcore" or lose their jobs.

He had already fired half of the company's 7,500 staff, scrapped a work-from-home policy and imposed long hours, all while his attempts to overhaul Twitter face backlash and delays.

"I may be #exceptional, but gosh darn it, I'm just not #hardcore," tweeted one former employee, Andrea Horst, whose LinkedIn profile still reads "Supply Chain & Capacity Management (Survivor) @Twitter."

His stumbling attempts to revamp user verification with a controversial subscription service have led to a slew of fake accounts and pranks, and prompted major advertisers to step away from the platform.

'Not super worried'

Fevered talk of the site's imminent demise was driving record high engagement on Twitter, according to Mr Musk.

He noted the irony by posting the popular meme of an actor jokingly posing over a grave. Both the man and the tombstone were overlaid with Twitter's logo. The post was "liked" by more than 1.3 million users.

In a later tweet, sent during Friday's early hours on the West Coast, the South African-born billionaire said: "Record numbers of users are logging in to see if Twitter is dead, ironically making it more alive than ever!".

Elon Musk added that the "best people are staying, so I'm not super worried."

Despite Mr Musk's assurances, entry to Twitter's offices were temporarily closed until Monday, even with a badge, according to an internal message seen on US media.

In the internal memo sent Wednesday, Musk had asked staff to follow a link to affirm their commitment to "the new Twitter" by 5:00 pm New York time (2200 GMT) on Thursday.

If they did not do so, they would have lost their jobs, receiving three months of severance pay.

Twitter did not respond to AFP requests for comment on the new measure.

Signs that government regulators were becoming impatient with Elon Musk's handling of Twitter also grew on Friday, especially over the platform's ability to moderate content with a severely reduced headcount.

A group of US senators on Thursday said Mr Musk's plans for the site "undermined the integrity and safety of the platform... despite clear warnings those changes would be abused for fraud, scams, and dangerous impersonation."

A top regulator for the European Union meanwhile said that Musk should be increasing the number of moderators in Europe, not reducing them.

Musk "knows perfectly well what the conditions are for Twitter to continue operating in Europe," EU commissioner Thierry Breton said.

A spokesman for German Chancellor Olaf Scholz said the government was watching developments at Twitter "with growing concern" and reviewing its presence on the platform.