Facebook is taking aim at people who are using the social network or Instagram photo-sharing platform to sell guns.

Under pressure from gun safety advocates, the social network will block members under 18 years of age from viewing pages or timeline posts reported to involve private sales of firearms and will set up online "checkpoints" warning people that such deals may be illegal.

And people offering guns for sale on Facebook will not be allowed to indicate that background checks are not required or that sales will 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.

The California-based company 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 praising Facebook's move.

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 leading social network's hardened policy will also apply to its smartphone photo sharing service Instagram, where gun sellers had taken to showing pictures of wares tagged with terms such as #gunsforsale.

Legal rules regarding gun sales vary from state to state, while 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 will work with advocacy groups to create a targeted ad campaign to teach social network members about gun laws.

Advocacy group Moms Demand Action for Gun Sense in America founder Shannon Watts said 230,000 people signed a petition as part of a campaign calling for Facebook and Instagram to crack down on gun sales at the social networks.

"American moms are gratified that Facebook and Instagram have agreed to take meaningful steps to prevent illegal gun sales to children and dangerous people on its platforms," Watts said in a release.