Hundreds of Twitter employees are estimated to have quit the beleaguered social media company following a Thursday deadline from new owner Elon Musk that staffers sign up for "long hours at high intensity," or leave.

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

Several Twitter employees had since made the decision to leave, highlighting the reluctance of some of its 3,000 or so employees that remain at the company to continue to work under what some have called Musk's draconian rule. The actual number of employees who have left is however unknown.

Musk had earlier laid off half of the company's 7,500 workforce including top management, and is ruthlessly changing the culture to emphasize long hours and an intense pace. Musk, himeself, took to Twitter late on Thursday and said that he was not worried about resignations as "the best people are staying."

Several reports suggest that the departure includes entire teams and several engineers responsible for fixing bugs and preventing service outages. This has raised questions about the stability and reliability of the platform going on.