Alec Baldwin says he assumed the gun he was holding was empty of live bullets before it went off and killed a cinematographer during a rehearsal on the set of the movie Rust.

In an emotional television interview which aired on Thursday night, US time, the actor also said he did not see any safety issues on the set before the incident where cinematographer Halyna Hutchins was killed.

"We both assumed the gun was empty, other than those, you know, dummy rounds," Mr Baldwin told the US ABC's George Stephanopoulos.

The actor had been handed the gun by a crew member and told it was safe.

He said he did not pull the trigger.

"I would never point a gun at anyone and pull a trigger at them," Mr Baldwin said.

"The notion there was a live round in that gun did not dawn on me until about 45 minutes later."

Ms Hutchins died and director Joel Souza was wounded when the gun fired off a live bullet.

David Halls, the assistant director who handed Baldwin the gun, backs up the actor's assertion that he did not pull the trigger.

Lisa Torraco, a lawyer for Mr Halls, told ABC earlier on Thursday that her client has always said Baldwin never pulled the trigger.

"He told me, since day one, he thought it was a misfire," Ms Torraco said.

"Until Alec said that, it was just really hard to believe, but Dave has told me since the very first day I met him, that Alec did not pull that trigger."

No criminal charges have been filed, but two crew members have filed civil lawsuits accusing Mr Baldwin, the producers and others of negligence and lax safety protocols.

"I did not observe any safety or security issues at all in the time I was there," Mr Baldwin said.

Authorities are focusing on how a live bullet, rather than a blank, ended up in the gun and how live rounds got onto the set at the Bonanza Creek Ranch.

Investigators have described "some complacency" in how weapons were handled on the Rust set.

They have said it was too soon to determine whether charges will be filed, amid independent civil lawsuits concerning liability in the fatal shooting.

Mr Baldwin said he met with the film's armourer, Hanna Gutierrez Reed, for a gun-training session before the shoot, and she appeared capable and responsible.

"I assumed because she was there and she was hired that she was up to the job," he said.

Ms Gutierrez Reed has been the subject of much of the scrutiny in the case.

Her attorney has said she did not put the round in the gun, and that she believed she was the victim of sabotage.

Reuters/AP