Officials inside Pulse Nightclub following the worst mass shooting in US history have been forced to try and tune out the sound of phones ringing as families and friends try to make contact with the victims still inside the venue.

Investigators at the scene were overwhelmed by the sounds of endlessly ringing phones coming from the bodies, as people continued to call hoping for their loved ones to answer, CNN reported.

CBS reporter Scott Pelley was reporting live from the scene when he too said he could hear the sounds of the victims’ phones ringing from inside the club.

One mother, Mina, is still waiting to hear news of her son, Eddie Justice, who sent her a series of harrowing texts during the shooting.

“He’s coming. I’m gonna die,” he wrote to her, saying he was in the club’s bathroom.

“He has us, and he’s in here with us,” he continued, before falling silent.

Mina has not heard from Mr Justice since, and his condition is unknown.

Mina Justice speaks to a reporter, discussing texting with her son Eddie Justice who was in a bathroom at Pulse during the shooting. (AAP)

Fifty people are dead and a further 53 injured, many in a critical condition, after gunman Omar Mateen opened fire inside the gay nightclub in Florida.

Ronald Hopper, FBI special agent in charge at the scene in Orlando, said Mateen, 29, was a US citizen born in New York to Afghan parents.

Omar Mateen. (Orlando Police/AAP)

Mateen shot his way into the Pulse nightclub at 2am on Sunday, and died in a shootout with police who stormed building about three hours later, authorities said.

After the shooting began, Mateen made telephone calls to 911 emergency services in which he made comments "general to the Islamic State" movement, Hopper said.

He would not directly confirm media reports that Mateen pledged adherence to the terrorist group.

Hopper said the FBI investigated Mateen in 2013, speaking to witnesses and interviewing him twice over reported statements in support of Islamist militants, but found no conclusive evidence against him.

In 2014, the FBI probed alleged ties between Mateen and a US citizen who became an Islamic State suicide bomber, including another interview, but agents determined that "contact was minimal", Hopper said.

Mateen was "not under current investigation at the time of this incident and was not under surveillance".