The Irish MMA pro, 33, appeared to yell at the singer as he walked the red carpet with girlfriend Megan Fox on Sunday, and McGregor had to be restrained by security guards, according to photos and video that captured the incident.
A source tells PEOPLE that McGregor, who is on hand as a presenter, asked to take a photo with Kelly, 31, but that his request was denied by the star's team.
"Conor went in on it a little bit," the source says, noting that the fighter dropped his cane, then retrieved it and began swinging it in Kelly's direction.
Kevin Mazur/MTV VMAs 2021/Getty Conor McGregor and Machine Gun Kelly
Jeff Kravitz/MTV VMAs 2021/Getty Conor McGregor
Kevin Mazur/MTV VMAs 2021/Getty Conor McGregor
A rep for Kelly did not immediately respond to a request for comment, and a rep for McGregor denied he asked for a photo.
For more on the 2021 MTV VMAs and other top stories, listen below to our daily podcast PEOPLE Every Day.
"Conor and Dee are in their seats enjoying the show. Conor appreciates all his fans," says McGregor's rep, adding that Machine Gun Kelly "attended Conor's last fight."
A video captured by a VMAs attendee and shared to Twitter appeared to show McGregor lunge toward Kelly and shout at him as security guards held him back.
Additional photos show the men standing face to face during the heated moment.
Kelly is scheduled to perform his new single "Papercuts" on the VMAs stage, while Fox, 35, is slated to present.
Inside the awards show, McGregor appeared to have moved on from the incident and was seen kissing his girlfriend Dee Devlin.
ANGELA WEISS/Getty Megan Fox and Machine Gun Kelly
The incident comes two months after McGregor was forced to undergo surgery after injuring his leg during a highly anticipated UFC 264 fight against Dustin Poirier in July.
The 2021 MTV Video Music Awards, hosted by Doja Cat, are airing live from New York City on Sunday, Sept. 12 at 8 p.m. ET.