Employee departures were multiplying at Twitter on Thursday after an ultimatum from new owner Elon Musk, who demanded staff choose between being "extremely hardcore" and working intense, long hours, or losing their jobs.

Musk, also the CEO of Tesla and SpaceX, has come under fire for radical changes at the social media company, which he bought for $44 billion late last month.

He had already fired half of the company's 7,500 staff, scrapped a work-from-home policy and imposed long hours, before taking to Twitter late on Thursday saying that he was not worried about resignations as "the best people are staying."

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.

The social media network's management told employees Thursday that offices were temporarily closed and inaccessible, even with a badge, according to Zoe Schiffer, a journalist for the tech industry newsletter Platformer.

"Going forward, to build a breakthrough Twitter 2.0 and succeed in an increasingly competitive world, we will need to be extremely hardcore," Musk wrote in the ultimatum, an internal memo sent Wednesday and seen by AFP.

"This will mean working long hours at high intensity. Only exceptional performance will constitute a passing grade," he added.

Staff had been asked to follow a link to affirm their commitment to "the new Twitter" by 5:00 p.m. ET (2200 GMT) on Thursday.

If they did not do so, they lost their jobs, receiving three months of severance pay – an unusual method even in the United States, where labor laws are less protective for employees than in many other developed countries.

Twitter did not respond to AFP and Reuters requests for comments on the new measure.

(With inputs from agencies.)