A woman broke down in tears as she spoke of her agonising wait to hear whether her son is dead or alive following the fatal shooting at an Orlando gay club.

Christine Leinonen's son Christopher is believed to have been at nightclub Pulse when suspected Islamic extremist Omar Mateen, 29, opened fire, killing 50 people and injuring 53 others.

Speaking to ABC News from outside the Orlando Regional Medical Centre, Ms Leinonen, 58, sobbed as she revealed she had been waiting hours for information on her son.

"I haven't heard anything. I have been here since 4 o clock in the morning. I have been waiting. I have been waiting by the emergency room, see if anybody gets called in.

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"They said there's a lot of dead bodies at the club and that's a crime scene and they can't identify anybody so it could be hours and hours before we find out.

"The hospital said there are some bodies at the hospital that came in and they died. They are not identifiable yet either. And that there a few who are in a critical conditions who aren't identified yet."

Ms Leinonen told the station she rushed to the hospital after reading a panicked Facebook post by one of Christopher's friends at around 3am Sunday - just an hour after Mateen started shooting.

Shortly before the attack, Mateen, who was born in New York to Afghan parents, called 911 and pledged allegiance to ISIS leader Abu Bakr al-Baghdadi, law enforcement officials told NBC News.

There were about 320 people inside the club at the time of the shootings and about 100 people were taken hostage. Mateen was killed in a shoot out with law enforcement officials at around 5am.

A friend who escaped the bloodbath told Leinonen he had seen Christopher's boyfriend being carried to an ambulance with multiple gunshot wounds but had not seen Christopher.

Ms Leinonen revealed how the last words she said to her son were 'I love you, Chris', following a conversation at 6pm Saturday - hours before the massacre.

She also urged politicians and Americans to come together against gun violence, describing victims as members of a "club that nobody wants to be in".

"Please could we do something with the assault weapons so that we could stop this club from ever getting any new members. I beg all of you please."

The gunman who opened fire inside a crowded nightclub here early Sunday morning, launching a rampage that killed 50 people and injured 53 others in the deadliest shooting spree in the country's history, had pledged allegiance to the Islamic State before the attack, according to US law enforcement officials.

She continued: "Please let's just all just get along... We are on this earth for such a short time. Let's try to get rid of the hatred and the violence, please."

Ms Leinonen added she had been "so proud" of her son for starting a gay-straight alliance in high school.