Irish mixed martial arts superstar Conor McGregor has been accused of sexually assaulting a woman at an NBA Finals in Miami last week, multiple US reports said Thursday.

ESPN and TMZ Sports cited letters from the woman's lawyer which said McGregor "violently" assaulted her in a bathroom at the Kaseya Center during the Miami Heat's game four defeat to Denver on Friday.

The 34-year-old UFC star is alleged to have "aggressively kissed" his accuser before attempting to force her into multiple sex acts.

McGregor denied the allegations in a statement emailed to AFP by his attorney Barbara Llanes.

"The allegations are false. Mr. McGregor will not be intimidated," Llanes said in the statement.

Police in Miami meanwhile would only confirm that its special victims unit was investigating a report filed on Sunday.

"This is an open investigation so no additional information can be released at this time," a Miami Police Department statement to AFP said without mentioning McGregor by name.

Both the Miami Heat and the NBA said they were aware of the reports and investigating.

"We are aware of the allegations and are conducting a full investigation," the Heat said in a statement.

"Pending the outcome of the investigation, we will withhold further comment."

McGregor's conduct at Friday's game had already come under scrutiny after an incident during a timeout where he punched a Miami Heat mascot after he appeared on court for a pre-planned promotional spot.

The person wearing the mascot costume was later taken to a local hospital for medical treatment before being released later that evening after receiving pain medication.

McGregor is one of the biggest global stars in the history of the hugely successful Ultimate Fighting Championship (UFC) mixed martial arts circuit, formerly holding the featherweight and lightweight titles.