NEW HARTFORD, United States (AFP) - A 17-year-old boy testified on Wednesday (Oct 21) to being punched and whipped with a cord during a deadly assault in a United States church carried out by his family and congregants.

The vicious Oct 11 beating at the Word of Life Christian Church left Christopher Leonard's 19-year-old brother Lucas dead and has plunged the small town of New Hartford, New York, into mourning.

The boys' parents, Bruce and Deborah Leonard - aged 65 and 59 - have been charged with manslaughter. Four other church members, including a half-sister of the brothers, are charged with second-degree assault.

All six pleaded not guilty.

Speaking very softly, at times almost inaudible, Christopher told how the pastor, who has not been charged, asked the brothers to stay behind after a lengthy Sunday service because of "what we had done".

Their parents, half-sister Sarah Ferguson, a stay-at-home mother of four, and three other members of the church were present, he said.

The teenager said he was punched in the stomach and whipped with the black electrical cord while being restrained in an hours' long ordeal that lasted from Sunday evening until around 3am on Monday.

The brothers were beaten, he said, because they did not want to answer questions on what they had been doing.

With a shock of thick brown hair, and Harry Potter-style dark-rimmed spectacles, he looked much younger than his 17 years. He wore a grey fleece over a purple T-shirt and khaki combat pants, and sneakers.

He told how Lucas was given cardiopulmonary resusitation, refusing to look at his half sister, who sat just feet away dressed in an orange prison jumpsuit with a calligraphy tattoo etched on the back of her neck.

After the beating, Christopher said he threw up several times, found it difficult to breathe and lay on the ground with his eyes shut.

"They took me to the second floor and they made me a bed," he said. He slept "on and off", and the police came the next day.

Police said it took hours to track down Christopher, complaining that the church did not immediately volunteer his whereabouts.

Police say Lucas was beaten to death because he wanted to leave the church, a secretive congregation of around 40 adults and children in the town next to Utica, 400km north-west of New York City.

Lucas died the following day in hospital. Christopher was also treated and is now under supervision of child services.

Oneida County district attorney Scott McNamara told reporters that he "can't imagine the stress" Christopher was under in taking the stand in a packed courtroom to testify against his family.

A New York state police officer, who photographed Christopher on Oct 12, said he sustained cuts and bruises to his back, and marks and bruises on his legs and buttocks.

Wednesday's preliminary court hearing has given prosecutors 45 days to present their case to a grand jury, who will determine whether to indict the suspects.

The judge set possible bail for Ferguson at US$50,000 (S$69, 630). The last suspect in custody, she sat sniffling throughout her brother's testimony.

Asked if he fought back, Christopher said he had no weapon. "I held up my hands," he said.

"Were you covering yourself?" asked assistant district attorney Dawn Lupi. "Yes," he replied.

New Hartford, a small town of around 24,000, has been left shocked and saddened by the killings, and some have called for the church to be dismantled.

"Knock it down, they're killers," said a handwritten sign tied to a post outside the imposing red brick building. A notice on the gate said the church was closed "during our time of mourning".

Helium balloons were left on the doorstep, and bouquets of flowers under a nearby tree with a sign saying "get well Chris" and "RIP Luke".

Ms Cindy Coch, a member of the local community who sells real estate, told reporters that the entire town was talking about the case, and that she was at the hearing to show support for the Leonard brothers.

"I just think it's awful that they refer to this place as a church; again, it's not a church," she told reporters outside the court.