A gunman attacked a school on Tuesday morning in the Russian city of Kazan, killing eight people — seven eighth-grade students and a teacher — and leaving 21 others hospitalised with wounds, Russian officials said.

Russian media said some students were able to escape the building during the attack, while others were trapped inside. Dozens of ambulances lined up at the entrance to the school after the attack, with access to the building fenced off by police.

Rustam Minnikhanov, governor of the Tatarstan republic where Kazan is the capital, said four boys and three girls, all eighth-grade students, died in the shooting. Minnikhanov's press service later said a teacher was also killed.

“The terrorist has been arrested, (he is) 19 years old. A firearm is registered in his name. Other accomplices haven't been established, an investigation is underway,” Minnikhanov said after visiting the school, adding that security had been restored.

Authorities said additional security measures were immediately put into place in all schools in Kazan, a city 700 kilometres (430 miles) east of Moscow. They also announced a day of mourning on Wednesday to honour the victims of the shooting.

According to Tatarstan health officials, 21 people were hospitalised with wounds after the attack, including 18 children, six of whom were in intensive care.

Russia's state RIA Novosti news agency reported earlier that 11 people had been killed in the Kazan shooting. There was no way to immediately reconcile the differing death

tolls. Police opened a criminal investigation into the shooting.

Russian President Vladimir Putin expressed his condolences to families of the victims and those injured in the shooting and ordered the government to revise gun regulations in light of the attack. Russia's Emergency Ministry has sent a plane with doctors and medical equipment to Kazan.

While school shootings are relatively rare in Russia, there have been several violent attacks on schools in recent years, mostly carried out by students.