ORLANDO –  A shooting at an LGBT nightclub in Orlando has resulted in a “mass casualty situation”, police have confirmed, adding that the attacker has been killed and a controlled explosion has been carried out at the venue.

Shots were fired at the Pulse Club, an LGBT nightclub in the city, in the early hours of Sunday. Staff at the venue posted on Facebook: “Everyone get out of pulse and keep running” and there were quickly reports on social media of an ongoing attack.

Orlando police said the shooting had resulted in mass casualties and urged people to stay away from the area. Local unconfirmed reports suggested as many as 20 people could have been injured in the attack, and video footage shared on social media showed a high police presence around the club.

Authorities said a controlled explosion had taken place inside the club, undertaken by law enforcement operatives, and urged media to avoid reporting inaccuracies. They also confirmed the attacker had been killed inside the club. 

The Orlando fire department bomb squad and hazardous materials team were in attendance at the scene, reported Associated Press.

One man who claimed he was inside the club said people were “screaming that people are dead”. “They are just pulling people out in stretchers loading them up,” he said. “Please god let everyone make it – tonight was supposed to be a fun night! One more minute and we would of been shot or worse.”

One witness told Sky News there had been more than 100 people inside the venue for the club’s Latin night when the gunman entered the building and began firing into the ceiling and into the crowd.

Richard Negroni said: “We just heard shots, it was less than a minute, it felt longer. There was a brief pause and we just ran. Everybody was just faces to the floor. I had someone over me, I really didn’t see [how many gunmen there were]. All I can tell you was the club was packed, there were over 100 people there, if the gunman came in through the front door people were injured or worse.”

Anthony Torres told Sky News he was outside the club when the shooting began and said he thought more than one person was involved in the attack. He believed the shooting lasted between 15 and 20 minutes.

The scene of the shooting at the Pulse nightclub in Orlando. Photograph: EPA

In a Facebook post, Negroni wrote that the shooter opened fire at about 2am and “people on the dancefloor and bar got down on the floor”. He said some people near the bar and back exit managed to escape quickly.

Local news organisation Wesh reported that a number of ambulances had been taken to the nearby hospital and people with injuries were being removed from the club. Many family members were still waiting to hear about the safety of their family members. Staff at the club posted a message on Facebook thanking people for their concern during the night’s “tragic event”.

“As soon as we have any information we will update everyone,” read the Facebook statement. “Please keep everyone in your prayers as we work through this tragic event. Thank you for your thoughts and love.”

The police are yet to issue a full statement, though a briefing will take place at 7am local time (12pm BST) with John Mina, chief of police, mayor Buddy Dyer, Orange County sheriff Jerry Demings and the FBI.

The attack comes just days after the fatal shooting of Voice contestant Christina Grimmie in Orlando, while she was signing autographs for fans after a show.

Kevin James Loibl has been named by police as the shooter in that incident and said the 26-year-old travelled to Orlando from his home in St Petersburg, Florida, about two hours’ drive away, with the intention of carrying out the crime. There is no known connection between Grimmie and Loibl.