NEW YORK - Thousands of guns are being offered for sale illegally by unlicensed firearms dealers on a single US website, according to a report released on Thursday by a group of US mayors campaigning for gun control.

Mayors Against Illegal Guns, founded and largely funded by New York Mayor Michael Bloomberg, examined sales on Armslist.com, a major gun-trading website that enables sales in all 50 states, over an eight-week period this year.

It said nearly a third of the ads posted were from "high-volume" sellers who lacked the federal license required of individuals who regularly sell firearms.

Private sellers are generally allowed to sell a gun legally online without checking the buyer's criminal record and mental-health history, although rules governing such sales differ from state to state. Anyone engaged "in the business" of selling guns is required by federal law to have a license and to perform background checks on buyers.

Based on its sample, the mayors' group estimated that nearly 240,000 guns would be advertized on the website in a year by high-volume sellers posing as private sellers who would not necessarily require background checks.

"These findings tell us that law enforcement, legislators and web sites all need to take steps to choke off this potentially deadly stream of illegal firearms sales," Bloomberg said in a statement.

He noted that the report was being released just two days before the anniversary of a mass shooting at an elementary school in Newtown, Connecticut, in which a gunman killed 20 children and six adults.

"Nothing has changed in Washington since Newtown," Bloomberg said.

In spite of outrage provoked by the Newtown shooting, a gun control bill endorsed by President Barack Obama stalled in the US Senate earlier this year, with gun rights advocates arguing that free access to guns is a constitutional right.

Gun control advocates, including some from Newtown, gathered at the National Cathedral in Washington on Thursday to mark this week's Newtown anniversary.

The "National Vigil for Victims of Gun Violence," attended by about 500 people, was organized by the nonprofit Newtown Foundation and the cathedral, both of which back a tightening of federal gun laws.

The Reverend Mel Kawakami of Newtown's United Methodist Church urged the activists to "work toward a world where there are no more school shootings.

"We gather to say, No more!" he said.

In September, the mayors' group estimated that as many as 25,000 guns a year may be sold on Armslist.com to people with criminal records.

Armslist.com offers a disclaimer that it "does not become involved in any transactions between parties" and its users must agree that they will not break the law. The website's operators did not immediately respond to a request for comment.

Bloomberg, one of the most prominent and wealthy advocates of tighter US gun laws, steps down as mayor at the end of the year, but has said he will remain active on the issue.