A US prosecutor said Thursday he would seek to press charges that carry the death penalty against a man accused of kidnapping three Ohio women, raping them and forcibly ending their pregnancies.

Ariel Castro, a 52-year-old unemployed bus driver, has already been charged with kidnapping and raping the women over the course of a decade in his home in Cleveland. He was ordered held on an $8 million bond on Thursday.

But Cuyahoga County prosecutor Timothy McGinty said investigators would also go back over the many torments Castro allegedly inflicted on the women during their long ordeal, with the aim of bringing even more serious charges.

"I fully intend to seek charges for each and every act of sexual violence, rape, each day of kidnapping, every felonious assault, all his attempted murders and each act of aggravated murder he committed by terminating pregnancies" for which he himself was responsible, McGinty said.

Castro is alleged to have beaten at least one of the women to terminate pregnancies that began while they were locked in his home, which McGinty called "a torture chamber and private prison."

McGinty warned that Ohio imposes the death penalty in cases of aggravated murder in the course of a kidnapping and said his office was "in a formal process in which we evaluate to seek charges eligible for the death penalty."

"The horrific brutality and torture that the victims endured for a decade is beyond comprehension," he said.

Castro was arrested on Monday after 27-year-old Amanda Berry managed to call out to a neighbor who kicked in the door to the suspect's home and rescued her and the six-year-old daughter she bore during her captivity.

Police arrived on the scene and entered the house, finding two more women, 23-year-old Gina DeJesus and 32-year-old Michelle Knight. All three had been snatched in separate incidents around a decade earlier.

On Thursday, Castro was presented to the Cleveland municipal court on charges of kidnapping and raping the three women, as well as kidnapping the child.

Assistant county prosecutor Brian Murphy said: "The charges against Mr Castro are based on premeditated, deliberate, depraved decisions to snatch three young ladies from Cleveland West Side streets to be used in whatever self-gratifying, self-serving way he saw fit," he said.

"Along with captivity, there were repeated beatings. They were bound and restrained and sexually assaulted, basically never freed to leave this residence."

Castro kept his head bowed impassively through much of the hearing, chewing on the collar of the dark-blue uniform issued to him during his detention.

The judge, Lauren Moore, set the bond terms -- $2 million in each of the four cases -- and instructed that Castro was to have no contact with any of his alleged victims.

After the hearing, public defender Kathleen DeMetz told reporters she believed Castro would be placed on suicide watch in jail.

Castro's mother Lillian Rodriguez briefly spoke to local media, saying: "I have a sick son who has committed something very serious. I'm suffering a lot. I ask for forgiveness from those mothers. May those young ladies forgive me."

Meanwhile, Cleveland media reported details of a note reportedly found by police in Castro's home in which he describes himself as a "sexual predator" and ponders killing himself and leaving his savings to his victims.

"They are here against their will because they made a mistake of getting in a car with a total stranger," said the note, which was apparently written in 2004 -- after Knight and Berry had already vanished.

Following this and other leaks, Cleveland Mayor Frank Jackson urged police officers and the media to respect the victims' privacy.

That three women could be held for so long in such a nondescript house in an unremarkable working class street has astounded local residents, and led to suggestions that Cleveland police had missed chances to find them.

But senior officers have defended the force, and underlined the careful steps Castro allegedly

took to conceal the women.

Castro has been described as a friendly neighbor who raised few suspicions but who also kept to himself, rarely if ever allowing anyone inside his home.

And one man who did visit him multiple times, local musician Ricky Sanchez, said he had seen no reason to be suspicious, though he noticed that the door had multiple heavy locks, and he once heard noises from the basement.

"I ask him for a minute, 'What is that? What is that noise?' 'Oh, that's just those dogs that I have down here'," Sanchez told AFP.

Late Thursday, Deborah Knight watched from a park at the end of the block where the three women were held captive as balloons were released to celebrate the release of her granddaughter Michelle.

Deborah Knight said she has not yet seen or spoken to Michelle, who remains in hospital following the harrowing ordeal.

"If I could speak to her and have her right next to me, I would give her the biggest hug and tell her I love her and missed her," she told reporters.