A US prosecutor warned Thursday that he will seek to press charges that carry the death penalty against a man accused of kidnapping three 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, Ohio. 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 Cleveland home.

One child, now a six-year-old girl, was born and was with the women when they were rescued on Monday.

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

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 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.

"Two of the victims incurred a horrifying ordeal for more than a decade, a third for close to a decade, and the ordeal eventually resulted in a little girl believed to have been born to one of the women while in captivity.

"And also, along with captivity, there were repeated beatings. They were bound and restrained and sexually assaulted, basically never freed to leave this residence," he said.

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: "I would imagine the county will place him in a single cell under a suicide watch and in protective custody where nobody has access to him."

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 when he had already captured the first two women.

Following this and other leaks, which described a series of rapes, beatings and forced miscarriages visited upon the women, 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, local musician Ricky Sanchez, said he had seen no reason to be suspicious.

"Let me tell you something, that guy was the nicest guy, you know, one of the nicest guys I ever met," Sanchez told AFP.

Sanchez said he visited the home more than 17 times, and had last week seen a young girl he was told was Castro's granddaughter. He had noted that the door had multiple heavy locks, and once heard noise 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.'"