ALBANY, N.Y. » Under pressure from gun control advocates, Facebook agreed Wednesday to delete posts from users seeking to buy or sell weapons illegally or without a background check.

A similar policy will be applied to Instagram, the company’s photo-sharing network, Facebook said. The measures will be put into effect over the next few weeks at the world’s largest social network, with 1.3 billion active users.

"We will remove reported posts that explicitly indicate a specific attempt to evade or help others evade the law," the company said in a statement.

The move reflects growing alarm that the Internet is being used to sell banned weapons, evade restrictions on interstate sales, and put guns in the hands of convicted felons, domestic abusers, the mentally ill or others barred under federal law from obtaining firearms. Gun control advocates say Facebook has become a significant marketplace, with thousands of firearms-related posts.

Google Plus and Craigslist already prohibit all gun sales, legal or illegal.

Facebook said that instead of patrolling its network for violators, it will rely on reports from users and police.

The new policy was worked out in an agreement with New York Attorney General Eric Schneiderman, who has been pressing the company along with Mayors Against Illegal Guns, a group backed by former New York Mayor Michael Bloomberg, and Moms Demand Action. Moms Demand Action collected more than 230,000 signatures on petitions calling on Facebook to act.

"Responsible social media sites know that it is in no one’s interest for their sites to become the 21st-century black market in dangerous and illegal goods that place our families and communities at risk," Schneiderman said.

Chris Cox, executive director of the National Rifle Association’s legislative policy arm, portrayed the new policy as a victory for the NRA, saying Bloomberg and his allies tried — and failed — to shut down discussion of gun rights on Facebook.

"NRA members and our supporters will continue to have a platform to exercise their First Amendment rights in support of their Second Amendment freedoms," Cox said.

But Tom King, president of the NRA’s New York affiliate, warned that the policy could be used to silence gun organizations on Facebook.

"This is something that could greatly get out of control very quickly," King said.

In crafting the new policy, Facebook had to confront a patchwork of gun laws around the country. New York, for example, has some of the nation’s toughest gun laws. It prohibits the sale of weapons such the AR-15 semi-automatic rifle, and it is one of 16 states that require background checks on buyers making private firearm purchases.

Under the new policy, Facebook would allow a user to list an AR-15 as long as it wasn’t offered for sale in states where the weapon is illegal.

Also, Facebook will remove any posts in which a gun seller offers to skip a background check, even if such checks aren’t required in the seller’s state. And it will delete offers to sell guns across state lines without the involvement of a licensed firearms dealer, something required under federal law.

"This is one of many areas where we face a difficult challenge balancing individuals’ desire to express themselves on our services and recognizing that this speech may have consequences elsewhere," Facebook said.

The company already has systems in place to remove advertising that is false and deceptive, and it prohibits ads for illegal drugs, tobacco and prescription drugs.

Dan Gross, president of the Brady Campaign to Prevent Gun Violence, complained that Facebook didn’t go far enough.

"They are talking about a community-based reporting system. Do what these other companies did and shut it down. Shut down the private sales of guns," he said.

There’s no way to know how many guns are sold via Facebook, because the transactions are actually completed offline, said John Feinblatt, chairman of Mayors Against Illegal Guns. But such sales have occurred.

In Kentucky, for example, federal authorities in February charged an Ohio man with illegally selling a 9 mm pistol to a Kentucky teenager in a transaction arranged through Facebook.

Feinblatt said that there are "virtual gun shows" online. His group issued a report in December showing 66,000 active ads on a popular gun sales website called Armslist.