Hamburg - An Afghan-born youth declined to testify Tuesday at his trial for the murder of his own sister in Germany.

The May 15 stabbing death of Morsal O, 16, in Hamburg attracted enormous attention at a time when Germans were concerned at a rash of "honour killings" by immigrants from archaic central Asian societies.

The term usually refers to a Muslim daughter being killed by the male members of her own family who collectively decide she has disgraced the family honour through a sexual relationship.

As more details of the Morsal case have emerged, some media have suggested the case may not fit that pattern.

Morsal and her elder brother were both troubled teenagers in the Hamburg family of an ethnic Tajik dealer in used trucks who had been a fighter pilot in the communist air force in Afghanistan.

The parents had difficulty managing both children.

Morsal was lazy at her schoolwork, but her parents were proud when she won a civic prize for setting up a peer counselling project at her school and her photograph appeared in newspapers.

The boy, Ahmad O, became a truant and a delinquent with a record of petty crime going back to 1998, including assaults on other members of his own family.

He was close to his sister, but objected to her wearing glitzy clothes and make-up. He stabbed her 23 times in the attack, one day after the schoolgirl had sought counselling from city youth aid officials.

Now aged 24, he told the court through his lawyer on the first day of trial Tuesday he would not testify on the charges.

German concern about honour killings came to a head with the 2005 murder of a Berlin woman of Turkish origin, Hatin Surucu, 23, by a brother after she fled an arranged marriage and stopped speaking to her relations.

The German organization Terre des Femmes, which campaigns for women's rights, stresses that honour killings do not have their origins in religion but in patriarchal social customs.

It estimates 50 women have been killed in the past decade in Germany for reasons of supposed honour.

In the Morsal case, the media have attacked the parents for taking Morsal out of school in 2007 and sending her to northern Afghanistan for nine months in a bid to teach her a more traditional way of life.

City authorities said they should have supervised the family more closely.

The muscular brother is alleged to have beaten up his sister on several occasions when she refused to follow his orders.

His lawyers reject the "honour killing" tag and say they will offer a defence of diminished responsibility for the stabbing, arguing that the brother was barely able to control his temper.

The case before the state court in Hamburg continues, with final arguments and a verdict scheduled for February. (dpa)