US media say Elon Musk has given workers at Twitter an ultimatum, asking them to work long hours at high intensity or leave the social media company.

The Washington Post and other media outlets on Wednesday reported on the ultimatum sent by Musk, who took over as Twitter's owner and CEO last month.

In a company-wide email titled "A Fork in the Road", Musk wrote, "Going forward, to build a breakthrough Twitter 2.0 and succeed in an increasingly competitive world, we will need to be extremely hardcore." He added, "This will mean working long hours at high intensity."

The employees who want to stay were asked to click "yes" on a link embedded in the email by 5 p.m. Eastern time Thursday.

The email said that anyone who has not done so will receive three months of severance pay.

Musk himself is known for working long hours.

The move comes after Twitter posted net losses due to declines in advertising revenue. Musk is apparently trying to achieve a turnaround by quickly implementing reforms.