Hundreds of Twitter employees are estimated to be leaving the company following an ultimatum from new owner Elon Musk that staffers sign up for "long hours at high intensity," or leave.

Early on Wednesday, Musk emailed Twitter employees, saying: "Going forward, to build a breakthrough Twitter 2.0 and succeed in an increasingly competitive world, we will need to be extremely hardcore".

The email asked staff to click "yes" if they wanted to stick around. Those who did not respond by 5 pm Eastern time on Thursday would be considered to have quit and given a severance package, the email said.

Inside the company's Slack, a mass resignation occurred after Musk's 5 pm deadline. Employees flooded the "#social-watercooler" channel with the salute emoji, indicating that they had chosen not to sign Musk's pledge.

It's a "mass exodus," CNN reported, quoting a former Twitter executive, who recently exited the company.

Musk was meeting some top employees to try and convince them to stay, one current employee and a recently departed employee who is in touch with Twitter colleagues told Reuters.

Musk also sent an all-staff email relaxing his previously uncompromising anti-remote work position.

"Regarding remote work, all that is required for approval is that your manager takes responsibility for ensuring that you are making an excellent contribution," Musk said in the email.

Last week, Musk told Twitter employees that remote work will no longer be permitted unless he personally approves it and that employees will have to be in a Twitter office for at least 40 hours a week.

Elon Musk completed his deal to buy Twitter at $44 billion on Oct 27, gaining control of the social network company.

Twitter on Friday alerted employees that all office buildings are temporarily closed and badge access is suspended. Offices will reopen on Nov 21, Bloomberg reported.