A kevlar helmet could have saved the life of an Orlando Police Department officer during the tense shoot-out between the police and gunman Omar Mateen at an Orlando nightclub on Sunday (June 12).

Special agent Danny Banks, who is in charge of the Florida Department of Law Enforcement's Orlando Bureau, told US media a bullet had struck the officer's helmet as police stormed the Pulse nightclub in downtown Orlando.

The officer suffered an eye injury but the helmet saved him from what could have been a fatal blow to his head.

A photo tweeted by the Orlando Police Department showed the helmet with a dented side and a bullet hole.

"In hail of gunfire in which suspect was killed, OPD officer was hit. Kevlar helmet saved his life," said the tweet, which has been retweeted over 12,000 times.

Orlando Police Chief John Mina said at least nine officers were involved in the shoot-out with Mateen, who had been armed with an AR15-type military assault rifle and a handgun.

They also used a "controlled explosion" to distract the gunman before fatally shooting him, and were able to rescue about 30 hostages who had been hiding in one of the club's bathrooms.

The 29-year-old shooter was said to be experienced in using firearms, having worked with prominent security firm G4S as an armed officer since 2007.

The massacre by Mateen, which killed 50 people and injured 53, is the worst mass shooting in US history.

Thus far, seven victims have been identified.

More information on their names and ages can be found on the City of Orlando website here.