Facebook is taking aim at people who are using its social network or the Instagram photo-sharing platform to sell guns.
Under pressure from gun safety advocates, Facebook will block members under 18 years of age from viewing pages or timeline posts that involve private sales of firearms and will set up online "checkpoints" warning people that such deals may be illegal.
People offering guns for sale on Facebook would not be allowed to say that background checks were not required or that sales would be done across state lines without involving licensed firearms dealers, it said.
"We will not permit people to post offers to sell regulated items that indicate a willingness to evade or help others evade the law," Facebook head of global policy management Monika Bickert said in a blog post.
Facebook said that it worked with New York attorney general Eric Schneiderman and advocacy groups to modify policies to fight illegal gun sales.
"Responsible social media sites know that it is in no one's interest for their sites to become a 21st century black market in dangerous and illegal goods that place our families and communities at risk," Schneiderman said in a statement.
Facebook and Instagram will remove content that represents a "direct, credible risk" to users and notify police when appropriate, according to Bickert.
Facebook's new rules also require people using social network pages to sell guns or other regulated items to display messages instructing buyers to obey applicable laws.
"By taking these unprecedented educational and enforcement steps, we've been able to strike an important balance in helping people express themselves, while promoting a safe and responsible community," she added.
The social network's policy will also apply to its smartphone photo sharing service Instagram, where gun sellers have taken to showing pictures of wares tagged with terms such as #gunsforsale.
Legal rules regarding gun sales vary from state to state. Federal laws apply to cross-border transactions. While Facebook and Instagram are not commerce websites, users are free to connect and buy or sell items to one another.
Facebook said it would work with advocacy groups to create an ad campaign to teach its users about gun laws.
Shannon Watts, founder of advocacy group Mums Demand Action for Gun Sense in America, said 230,000 people had signed a petition as part of a campaign calling for Facebook and Instagram to crack down on gun sales enabled by the social networks.