This picture made available to AFP by a student shows the masked man armed with a sword posing for a photo at the primary and middle school in Trollhattan, southwestern Sweden, on Thursday.

AFP/Trollhättan

A masked man brandishing a sword killed a teacher and a student and seriously wounded two other people at a school in Sweden on Thursday before being shot and killed.

Pictures taken by students and circulating in the media showed the assailant wearing black clothes and a Darth Vader-like mask, with school children initially thinking it was a prank or a Halloween costume.

The attacker, who went from classroom to classroom, struck at a school for six to 15-year-olds in the southwestern town of Trollhattan, shocking a nation where such violence is rare.

One teacher was found dead at the school - described in the media as a "problem school" - while a second victim and the assailant died in hospital several hours later.

Police investigator Thord Haraldsson confirmed the second victim was a young male student.

"The assailant knocked on two classroom doors and he attacked the two students who opened the doors," he said.

Assailant dies

Hospital staff had previously confirmed that two boys, aged 11 and 15, were in critical condition with stab wounds, while another teacher who was seriously wounded in the attack was also undergoing surgery.

The assailant was a 21-year-old man from Trollhattan who was shot by police at the scene and later died from his wounds in hospital.

Police were alerted to the incident at 10:10 am and have already identified the attacker.

However they have not disclosed any further information about him and the motive for the attack remains unclear.

"When we first saw him, we thought it was a joke. He was wearing a mask and black clothes and (carrying) a long sword. Some students wanted to take their picture with him and feel the sword," one student identified only as Laith told Swedish television SVT.

When the man started attacking people, he quickly realised it wasn't a joke and fled as the assailant went from classroom to classroom looking for victims.

Another student, 14-year-old David Issa, told AFP he was sitting in the school's cafe when he saw the attacker approach.

'Black day for Sweden'  

"We were sitting in the (school's) cafe and then this guy came up who was wearing a mask and carrying a sword and he stabbed my teacher. I panicked and ran away," he said.

"Then the police came. And he started stabbing others in the classrooms, banging on the classrooms and stabbing people in there," he said.

Aster Caridad, a 15-year-old student at the school, said one of the two wounded students was her friend.

"The teacher ordered us not to leave the classroom as someone was murdered and others (were) injured," she said.

"I never expected or even imagined this could happen in my school," she added.

Police said several knives were used in the attack.

They said there was "a lot of confusion" at the school, and pupils and teachers remained inside the building several hours after the attack.

According to its website, the Kronan school has around 400 pupils, including many newly-arrived immigrants.

'Problem school'

Swedish media described it as a "problem school".

It had been criticised by the Swedish Schools Inspectorate for its lax security and on account of a number of disruptions which prevented the pupils from learning.

Teachers had complained to the inspectorate about the school library and cafe being open to the public and creating an insecure environment for the children, Swedish news agency TT reported.

Swedish Prime Minister Stefan Lofven was to visit the town on Thursday.

"This is a black day for Sweden," he said in a statement.

"My thoughts go to the victims and their families, the students and staff, and the entire community. No words can describe what they're going through right now."

Crisis teams were at the school assisting parents and students.

Trollhattan is an industrial town of 57,000 and home to the former Swedish carmaker Saab which filed for bankruptcy in 2012.