Police in Florida are investigating an early morning fire at the mosque attended by Omar Mateen, the Orlando nightclub shooter.

No-one was injured in the blaze at the Islamic Centre of Fort Pierce.

The St Lucie County Sheriff’s Office said surveillance video showed someone approaching the building moments before a flash is seen and the fire began.

“A fire at any place of worship is alarming, regardless of the circumstances,” it said on Facebook.

The fire was extinguished by firefighters after a 911 call was made. The full extent of the damage has not yet been determined.

The mosque was used by Mateen, the gunman who opened fire at the Pulse nightclub in June, killing 49 victims.

The rampage in the gay club was the worst mass shooting in modern American history.

Who is Omar Mateen?

Major David Thompson, of the St Lucie County Sheriff's Office, declined to speculate on whether the date of the fire, at the end of the 15th anniversary of the September 11 terrorist attacks, was significant but he confirmed that investigators were treating it as arson.

“Evidence has revealed that this fire was set intentionally,” he said

Multiple agencies, including the State Fire Marshal's Office, the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and the FBI, are investigating.

Firefighters at the Islamic Centre of Fort Pierce, Florida (AP)

Sheriff's officials added that they planned to release the surveillance footage and will appeal to the public for help in identifying the person responsible.

It's not the first time the mosque has seen trouble since the Orlando attack.

A few weeks after the shooting, a man was beaten outside the mosque, according to Sheriff Ken Mascara. Early on July 2, deputies were called by a man who said someone was trying to break into a vehicle.

Deputies found a man bleeding from the mouth who told them he was approached by someone who “asked him what he was doing and then punched him several times in the face and head”.

The man was stopped and arrested a short time later.

The Council on American-Islamic Relations said in a statement that a man in a truck had stopped outside the mosque earlier that day and made slurs, including, “you Muslims need to get back to your country.”