The Alternative for Germany (AfD) party and other far-right groups have seized on the brutal killing of 14-year-old Susanna Maria Feldman in their campaign against Chancellor Angela Merkel's asylum policy.

The accused, rejected asylum seeker Ali Bashar, 22, was convicted of the killing and handed the maximum sentence of life in prison by the court in Wiesbaden, the city where the murder took place in May last year.

The judge, Juergen Bonk, also found the crime to be of exceptional severity, meaning that Bashar will not be granted parole after 15 years as is usually the case in Germany but kept behind bars.

Bashar battered, raped and strangled the schoolgirl to death in a wooded area near railway tracks on May 23.

The court heard he then sent false messages from Susanna's smartphone indicating she had left for an impromptu trip to Paris.

Her body was only found on June 6 in a shallow grave covered with leaves, twigs and soil.

By this time, Bashar and his family had left Germany and returned to Arbil in northern Iraq.

The accused was however arrested by Kurdish security forces and, even though Berlin and Baghdad have no formal extradition treaty, taken back to Germany.

- Incapable of empathy -

Federal police chief Dieter Romann personally joined the controversial operation, as newspaper front pages showed pictures of commandos escorting a heavily restrained Bashar off an aircraft.

Bashar later confessed the killing but denied the rape, claiming that he and the girl had consensual sex before she fell, got angry and threatened to call the police.

In a separate trial, Bashar is accused of twice raping an 11-year-old girl, who was believed to have also been sexually assaulted by an Afghan youth.

The Bashar case put renewed pressure on Merkel's government over the decision to keep open German borders at the height of Europe's refugee crisis.

The far-right has voiced fury about cases of sexual violence committed by recent migrants and other foreign nationals -- including mass assaults in Cologne on New Year's Eve 2015-16, and, in April this year, the alleged gang rape by eight Bulgarian teens of a 13-year-old girl.

Bashar, his parents and siblings arrived in Germany in 2015, at the peak of the influx that would bring more than a million asylum seekers, mostly from the Middle East and Africa.

His asylum request was rejected in late 2016, but he obtained a temporary residence permit pending his appeal.

During this time, he came to police attention for fights, alleged robbery and possession of an illegal switchblade.

Susanna's mother, Diana Feldmann, dressed in black during the almost four-month-long trial, broke down in tears as the court heard the harrowing details of the crime.

"I have already been given a lifetime sentence," she told a court last week.

Bashar by contrast appeared composed during the trial, in which a psychiatrist testified the accused has a personality disorder and is incapable of empathy.