An eyewitness video appears to shows the moment at least one gunman opened fire during a music festival near Las Vegas' Mandalay Bay casino, leaving at least two dead.

The video shows people enjoying a performance at the Route 91 Harvest Festival, near the casino, before the sound of gunfire can be heard. Witnesses say country singer Jason Aldean was playing near the end of the concert when gunfire rang out.

Two festival-goers reported hearing a noise "like firecrackers" before realising a shooter or shooters were unloading "clip after clip" into the crowd. One man insisted there were "multiple shooters" involved.

The footage shows festival-goers fleeing the area amidst more gun shots, and then the screaming starts. One person in the video is heard crying.

Police have placed the area on lockdown and closed off parts of the Las Vegas strip and Interstate 15.