Facebook has unveiled a "safety check" tool to allow users to alert friends and family about their status in emergencies, a system developed after the 2011 Japan earthquake and tsunami.

"We want to provide a helpful tool that people can use when major disasters strike, so we've created 'safety check' - a simple and easy way to say you're safe and check on others," a Facebook blog post said late yesterday. Facebook said it began working on the system after the Japan disaster: "During that crisis, we saw how people used technology and social media to stay connected with those they cared about," the statement said.

The new tool expands on its "disaster message board" used in Japan. In "safety check," users in areas affected by a disaster will get a notification asking "Are you OK?" Users can select to click "I'm safe" and status updates will be sent to friends.