A gunman has shot dead "around 20 people" in what authorities have described as "an act of terrorism" on a gay nightclub in Florida.

The shooter, carrying a device thought to be a bomb and armed with an assault rifle and a handgun, opened fire in the Pulse Orlando bar in the early hours of Sunday morning before taking hostages.

The attacker, who was said by the FBI to have "leanings" towards extremism and Islamic State, was involved with a three-hour stand-off with officers before SWAT teams stormed the building and shot him dead.

Speaking at a press conference after the attack, FBI Special Agent Ron Hopper said: "Unfortunately there are people who died from gunshot wounds, maybe around 20, inside the nightclub."

Some 42 people are being treated for injuries in hospitals in Orlando.

Law Enforcement Agent Danny Banks described the attack as an "act of terrorism" and said: "Whether that is domestic terrorist activity or an international one, that is something we will certainly get to the bottom of."

He said they were trying to establish whether the attacker was a "lone wolf" or had accomplices.

However, Special Agent Hopper insisted there was no indication there would be further, similar attacks.

Barack Obama was briefed on the shooting by his counterterrorism aide this morning and a White House statement said: "Our thoughts and prayers are with the families and loved ones of the victims."

The attack unfolded just after 2am when an officer working as a security guard at the club raised the alarm and engaged in a gunbattle in the street with the assailant. The attacker ran back inside the club where he took hostages.

Some clubbers mistook the sound of gunshots for music and the bar posted on its Facebook account: "Everyone get out of pulse and keep running."

At 5am police launched a mission to rescue the hostages and nine officers stormed the club, shooting dead the attacker following a short gunfight.

. @orlandomayor Our community is strong. We will need to help each other's get through this. pic.twitter.com/XyUa8g5PT8
— Orlando Police (@OrlandoPolice) June 12, 2016

One officer was shot in the head but his Kevlar helmet saved his life.

Ricardo Negron was in the Pulse club at the start of the attack and told Sky News: "At around 2am someone started shooting. People just dropped on the floor. I guess the shooter was shooting at the ceiling because you could see all the glass from the lamps falling.

"There was a brief pause in the shooting and some of us just got up and ran out the back."

Christopher Hansen said he was in the VIP lounge when he heard gunshots.

"I was thinking, are you kidding me? So I just dropped down. I just said please, please, please, I want to make it out," he said.

"And when I did, I saw people shot. I saw blood. You hope and pray you don't get shot."

The shooting comes after Christina Grimmie, a star of The Voice, was shot dead by a deranged fan as she performed at the Plaze Live venue in Orlando on Friday.

Up to 20 killed and 42 wounded in Florida nightclub shooting
• Omar Mateen, a US citizen from Port St. Lucie, Florida identified as gunman
• Police storm Pulse nightclub after standoff and release 30 hostages
• FBI believe shooting was terrorism
• Customers on Facebook told to "get out of Pulse and keep running"
• Gunman confirmed dead

....who was said by the FBI to have "leanings" towards extremism and Islamic State

General Mackevili wrote:Bomb? 20 innocent people dead? Islam?

I feel like I've read this before, which is strange because these words are rarely found in the same sentence.... hmmmmm