A special needs teacher and her eight-year-old student have died after a gunman opened fire at a school in California.

Karen Smith was teaching a class when she was murdered by her estranged husband, Cedric Anderson, in San Bernardino.

Two children were caught in the gunfire unintentionally because they had been standing behind their teacher.

One of the students is said to be in a stable condition, but the other - named by police as Jonathan Martinez - died from his injuries in hospital.

The shooter turned the gun on himself after the attack at North Park Elementary School, and investigators are treating it as a "murder-suicide".

Officials say Anderson (53) had a criminal history including domestic violence and weapons charges.

San Bernardino police chief Jarrod Burguan said Anderson had been married briefly to Ms Smith, but they had been separated for about a month and a half.

Karen Smith, 53, seen in this photo provided by the San Bernardino Police Department, was killed Monday in a school shooting in San Bernardino | Image: USA TODAY Network/SIPA USA/PA Images

Moments before the shooting, Anderson had been welcomed into the school as a legitimate visitor - and he had told a receptionist that he was "dropping something off with his wife".

The gunman had kept his large-calibre revolver concealed until he was in the classroom. Fifteen students and two teaching assistants are believed to have witnessed the attack.

Following the shooting, hundreds of students were escorted to a nearby campus.

"They're being well cared for - having snacks, playing games and watching a Disney movie," one officer tweeted, as concerned parents descended on the elementary school.

One concerned mother, whose nine-year-old son and seven-year-old daughter attend North Park, described the incident as "every parent's worst nightmare".

The gate of North Park Elementary School is sealed off in San Bernardino of California | Image: Xinhua/SIPA USA/PA Images

The shooting took place about eight miles from where a radicalised Muslim couple killed 14 people during a terrorist attack in December 2015.

Husband-and-wife attackers Syed Rizwan Farook and Tashfeen Malik died in a gun battle with authorities shortly afterwards.

There are dozens of cases of gun violence on school campuses every year in the US.

Overall, more than 10 in every 100,000 Americans are killed by guns - a rate which is far higher than in other Western countries.

In February, US President Donald Trump signed a measure into law which blocked an Obama administration rule designed to stop people with certain mental disorders from being able to purchase firearms.

The policy had been proposed by Barack Obama in the aftermath of the Sandy Hook massacre in 2012, when a young man with Asperger's syndrome shot dead his mother, six teachers and 20 children before turning the gun on himself.