Thousands of guns are being offered for sale illegally by unlicensed firearms dealers on a US website, according to a report released by a group of mayors campaigning for weapons control. Mayors Against Illegal Guns, founded and largely funded by New York mayor Michael Bloomberg, examined sales on Armslist.com a leading gun-trading website that enables sales in all 50 US states, over an eight-week period this year. It said on Thursday that nearly a third of the ads posted were from "high-volume" sellers who lacked the federal licences required of individuals who regularly sold 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 licence and to perform background checks on buyers. Based on its sample, the mayors' group estimated that nearly 240,000 guns would be advertised on the website in a year by high-volume sellers posing as private sellers who would not require background checks. "These findings tell us that law enforcement, legislators and websites all need to take steps to choke off this potentially deadly stream of illegal firearms sales," Bloomberg said. He noted that the report was being released just two days before the anniversary of last year's mass shooting at an elementary school in Newtown, Connecticut, in which a gunman killed 20 children and six adults.