A masked man brandishing a sword has attacked a school in Sweden, killing a teacher and a student before being shot and killed by police.

The ambush took place in a school for children aged between six and 15 in the south-western city of Trollhattan.

“One teacher has died at the scene,” local health officials said, indicating that another teacher and two children, aged 11 and 15, were also wounded in the assault.

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The second victim was either the 11-year-old or the 15-year-old, but it had not been confirmed which one.

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

Police said they shot the assailant and he later died in hospital.

Authorities were alerted to the attack at around 10:10am local time. The attacker’s motive was not immediately clear.

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.

People light candles outside the school as a tribute to the victims. Photo: Getty

“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 unidentified student told news agency TT.

When the man started attacking people, the student quickly realised it was not a joke and fled as the assailant went from classroom to classroom looking for victims.

TT reported several knives were used in the attack.

Police said there was “a lot of confusion” at the school, saying there were still pupils and teachers inside more than two hours after the attack. The Kronan school has around 400 pupils.

Swedish prime minister Stefan Lofven was expected to visit the town later on Thursday, the Swedish parliament’s Twitter account said.

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

School attacks are rare in Sweden. A 1961 school shooting in Kungalv, in south-western Sweden, left one person dead and six others injured.

No other mass shootings have occurred since then, though at least one attack has been foiled, in the southern city of Malmo in 2004. Other threats have been issued but not followed through.

– with AAP