A teacher and a pupil have been killed in a sword attack at a school in western Sweden.

Police said they were called to reports of a masked attacker in the cafe area of the Kronan school in Trollhattan, north of Sweden’s second-largest city Gothenburg.

The male teacher was pronounced dead at the scene, while another teacher and two pupils, boys aged 11 and 15, were found with severe injuries.

One of the boys died in hospital a short time later.

The other victims "hover between life and death", a doctor at the hospital told reporters.

The attacker is also in a serious condition after he was shot by police.

Police spokesman Thomas Fuxborg said the suspect, who is in his 20s, carried more than one weapon, including "at least one knife-like object".

He said police fired two shots, one of which hit the masked man.

"We thought it was a joke"

Terrifying accounts of the attack are now starting to emerge, with one youngster saying she initially thought the attacker was dressed up for Halloween.

"We thought it was a joke, some sort of a Halloween prank but it wasn't," 14-year-old Edona said.

She said she realised what was happening when other pupils around her started to cry and panic.

Another teenager told Sweden's English-speaking news site The Local he was forced to run from the attacker after his teacher was stabbed in front of him.

"I was in a classroom with my class when one of my classmates’ sisters called her to warn her that there was a murderer at the school," he said.

"So we locked the door to the classroom, but our teacher was still outside in the corridor".

"We wanted to warn him, so a few of us went outside and then I saw the murderer, he was wearing a mask and had a sword. Our teacher got stabbed".

"The murderer started chasing me, I ran into another classroom".

"If I had not run, I would have been murdered. I’m feeling really scared. Everyone’s scared here".

The injured were taken to the nearby NAL hospital in Trollhattan, where the three victims had surgery.

Dr Lars Spetz told reporters the wounded teacher had been stabbed in the abdomen, while the boys were stabbed in the abdomen, liver and chest.

The attacker had gun wounds to his lower chest, he said.

The motive for the attack remains unclear. It is also not yet known whether the suspect had any ties to the school.

Swedish Prime Minister Stefan Lofven, who is on his way to Trollhattan, has said today is a "black day" for Sweden.

"My thoughts go out to the victims and their families, the students and staff, and the whole community that has been affected," he said.

"No words can describe what they are going through right now".

Some 400 pupils, ranging from pre-school to high school, attend the school.