The man responsible for the worst mass shooting in US history calmly pledged allegiance to Islamic State and grew increasingly angry as police tried to keep him on the phone, recordings reveal.

Omar Mateen massacred 49 people at a gay nightclub in Orlando, Florida in June before being shot dead by SWAT teams after a three-hour stand-off.

While the transcripts of what the 29-year-old told police have previously been released, this is the first time the voice of the killer during his rampage has been heard.

A judge has ruled they should be made public, and more could be released in the future.

The recordings - which can be downloaded on the Orlando police website - reveal that police negotiators at first were not sure if the person they were speaking to was actually in the Pulse nightclub.

They capture police officials strategising among themselves about how to talk to Mateen, who hung up a number of times.

In one of the calls to police, Mateen, a US citizen of Afghan descent, calmly states: "You're speaking with a person who pledged his allegiance to the Islamic State of Abu Bakr al Baghdadi and Allah."

He then hits out at US bombings in Iraq and Syria, saying: "They're killing a lot of innocent people, so what am I to do here when my people are getting killed over there? You get what I'm saying?"

When Mateen is asked what is going on at the nightclub, he replies: "What's going on is that I feel the pain of the people getting killed in Syria and Iraq."

He praises his "homeboy", Boston marathon bomber Tamerlan Tsarnaev, and adds: "So now it's my turn, OK?"

As a police negotiator, who identifies himself as "Andy", asks Mateen questions, he grows agitated.

"Look, you're annoying me with a lot of your phone calls. I call you."

The police's attempts to get any information out of Mateen proved unsuccessful, with him hanging up a number of times.

Efforts to get Mateen to release the hostages he had taken during his rampage also failed, and police eventually took the decision to storm the nightclub.

After a short gunfight, Mateen was shot dead.