At least 20 people are dead in a shooting in a LGBTI nightclub in Orlando, Florida, which police labelled an act of "domestic terrorism".
Patrons at the Pulse nightclub were attending a Latin-themed night when the gunman opened fire around 2am on Sunday.
A number of people were shot and patrons fled the club.
An officer working at Pulse nightclub then became engaged in a "gun battle" on the street outside the building with the suspect, who then returned inside where more shots were fired and hostages taken.
Chief John Mina said that there was a three-hour stand-off before SWAT teams stormed the building and shot dead the gunman.
Addressing the media, police said that the suspect appeared to be carrying an assault-type rifle, a handgun and "some type of device on him".
A bomb disposal squad and sniffer dogs were observed at the scene.
Police said they had carried out a "controlled explosion" at the club hours after the shooting broke out, but did not say why that was done. They described the scene as a "fluid situation".
Police said that in addition to the approximately 20 people who were killed, at least another 42 were injured and taken to hospital.
The FBI is investigating the attack.
Pulse Shooting: The shooter inside the club is dead.— Orlando Police (@OrlandoPolice) June 12, 2016
We can confirm this is a mass casualty situation. Support from local/state/federal agencies. We expect to brief media shortly.— Orlando Police (@OrlandoPolice) June 12, 2016
Police set up a cordon around the club and urged people to stay away from the area.
Shooting at Pulse Nightclub on S Orange. Multiple injuries. Stay away from area. pic.twitter.com/5Di2mc6XUY— Orlando Police (@OrlandoPolice) June 12, 2016
Pulse, which is an LGBT nightclub, posted a message to its Facebook page which read: "Everyone get out of pulse and keep running."
A local TV news reporter, Stewart Moore, tweeted that a source had told him the suspect is wearing a bomb.
While a witness has told Sky News that he saw more than one shooter.
"Everyone was having a great time when it happened," he said.
"As we left the club we started hearing shots and we managed to make it to a convenience store down the road."
The New York Daily News has reported that one witness saw "at least two assailants open fire with what sounded like a semi-automatic firearm inside the nightclub and outside the south Orlando venue from a vehicle."
On Friday, Christina Grimmie, a US The Voice singer, was shot dead during an autograph session at a concert at The Plaza Live in Orlando.
#BREAKING: More than 20 people are reportedly being held hostage after a shooting at a nightclub in Orlando, USA.— Sky News Australia (@SkyNewsAust) June 12, 2016
Orlando mass shooting update from Pulse FB page. pic.twitter.com/a20s0VTrBR— Erick Weber (@ErickWeber) June 12, 2016
This is a developing story.