WINNENDEN, Germany - Germany was in mourning Thursday after a teenage gunman ran amok at a school and the local area, killing at least 15 people and finally himself in a shootout with police.

Seventeen-year-old Tim Kretschmer Wednesday burst into his former secondary school in Winnenden, a picturesque town in southern Germany, and opened fire, leaving nine pupils aged between 15 and 16 and three teachers dead.

Another three bystanders also died during a massive manhunt involving hundreds of armed police commandos and snipers in black body armour, assisted by helicopters and dogs.

Less than three hours after the killing spree began, the gunman eventually turned his weapon on himself after being injured in a fire-fight with police, bringing the death toll to 16.

A visibly shaken Angela Merkel, Germany's Chancellor, described the crime as "incomprehensible".

"It is a day of mourning for the whole of Germany. Our thoughts go out to the families and the friends. We are thinking of you and we are praying for you," she added.

Interior Minister Wolfgang Schauble ordered flags across Germany to be flown at half mast Thursday as a mark of respect for the slain. Church services took place Wednesday in the town, which has a population around 27,000.

The slaughter started at 9:30 am (0830 GMT) when Kretschmer entered the school he left last year, pulled out his nine-millimetre pistol and began his bloody rampage.

"He was constantly reloading his weapon," local police chief Konrad Gelden told reporters.

"He just opened the door, pulled out his gun and started shooting," one pupil said on German television. "One person saw someone shot in the head."

"My brother was in the classroom in which it happened," said another. "He was sitting next to his girlfriend and she was shot."

Kretschmer went into classroom 10d three times, the Bild daily said on its website, hissing on the third visit: "Aren't you all dead yet?"

A teacher threw herself in front of a female pupil -- and was shot by the gunman, Bild said.

All but one of the nine pupils killed at the school were girls, and the three teachers were women, said Heribert Rech, interior minister of the southern state of Baden-Wuerttemberg where the slaughter took place.

"I don't want to speculate too much about this at this point ... The pupils sitting nearest the door were girls," he said.

However, officials said the killer apparently had no motive for the bloodbath and there were no indications that he held a grudge against the school.

"The gunman wanted to destroy an entire school," Rech told reporters. "He was completely unremarkable."

Kretschmer had achieved "average" marks in his school-leaving certificate and then enrolled in a course to be a salesman, Rech said. He also regularly worked out at the gym and belonged to a sports club.

The school, which has 600 pupils, was part of a complex of several other schools with a total of 1,700 students aged from six to 19.

Wednesday's massacre was one of the worst school shootings in Germany in recent years.

In April 2002, 19-year-old Robert Steinhaeuser, a disgruntled student from Erfurt in eastern Germany who had been expelled, killed 16 people before turning the gun on himself.

In November 2006, a former student at a vocational school in Lower Saxony in northwestern Germany went a shooting spree in the establishment, injuring 37 people before killing himself.

The German school shooting came just hours after a gunman went on the rampage in the southern US state of Alabama, killing 10 people before also turning the gun on himself.

In the late afternoon, eight hours after the first shots were fired, the town, bathed in weak sunlight, still seemed to be in a state of shock.

Only policemen in green uniforms were still on the streets, the commandos in black camouflage long since back in their barracks.

After the screams and police sirens that had broken the calm of this picturesque southern German town, silence reigned.