YouTube has been forced to fix a flaw allowing hackers to bombard users with fake pop-up messages and redirect them to adult sites.

Hackers placed code in the comments section, under targeted videos, that would run when people watched the clip.

In some cases, a pop-up screen appeared reporting that the Canadian singer, Justin Bieber, had died in a car crash.

Google, which owns YouTube, said that it had fixed the problem "about two hours" after it was discovered.

"We took swift action to fix a cross-site scripting (XSS) vulnerability on youtube.com," a spokesperson said.

"Comments were temporarily hidden by default within an hour, and we released a complete fix for the issue in about two hours.