A US father is facing a second-degree murder charge for allegedly beating a man who tried to enter a petrol station toilet that his daughter was using.

Melvin Harris, 40, was arrested after the 2 August incident in Arizona and charged with assault after he confessed to attacking the man, police say.

The charge was upgraded to second-degree murder after the victim died of his injuries five days later.

Mr Harris’ fiancee told local media he was trying to protect his daughter.

According to a police report obtained by KPHO-TV, the attack happened at a petrol station in Phoenix, Arizona, where Mr Harris stopped off while picking up his 16-year-old daughter and her two friends just before midnight.

The report states that the alleged bathroom attacker, named in local media as Leon Leevon Armstrong, had encountered Mr Harris moments earlier when he had asked the father for money.

Mr Harris told police that he had given the man some money and then watched him go into the QuikTrip convenience store at the petrol station.

His daughter then emerged and said that the man had tried to force his way into the toilet stall that she had been using.

According to the police report, Mr Harris went into the shop to notify a security guard, telling him “he needed to take care of the situation or he’d do it himself”.

He went to drive away, but pulled over the car after spotting Mr Armstrong outside the shop.

Mr Harris then punched the man in the face, causing him to fall to the ground, according to the report.

According to witnesses, he continued to punch and kick Mr Armstrong as he lay on the ground, but Mr Harris told police that he does not recall continuing to attack him after he fell to the ground.

Using surveillance footage, police were quickly able to locate Mr Harris at his home, where he confessed to attacking the man.

But he told police that Mr Armstrong had swung at him first.

According to the Maricopa Sheriff’s office, Mr Armstrong has a criminal history of shoplifting, trespassing and obstructing police.

Mr Harris’s fianc that she didn’t “feel bad at all for his actions”, but that she felt bad Mr Armstrong “ended up dying in the process”.

“I’m not mad at him, I don’t feel like he did anything wrong,” Diana Jackson told the station.

“He did what he was supposed to do for our kid. You cannot tell someone they’re wrong for protecting their children,” she said.