Facebook has posted an explanation as to why some users have been experiencing issues with viewing photos on the social networking service since Friday.

"You may have noticed in the past day that some photos aren't appearing or are displaying a "question mark" graphic when you go to view them", an engineer said on the official Facebook blog. "We have experienced some problems with our photo storage that affected between 10 to 15 percent of already uploaded photos".

According to Facebook, the issue occurred after a "routine software upgrade on Friday night [where] a few of the hard drives where we store photos apparently failed all at once".

Facebook says users shouldn't worry however: "We still have all your photos because we store them in a way that maintains multiple copies of the data in case of hardware failures like this.

"We're restoring photos as we repair the hard drives", said the Facebook team before confirming that they "should be back to normal by early next week".