At least four people were injured in a shooting at a high school north of Los Angeles on Thursday, triggering a police hunt for the suspect who was later taken into custody.

Police and ambulances swarmed the area around Saugus High School in Santa Clarita, 65km north of Los Angeles, after the early morning shooting sent panicked children running from the area.

"Update regarding the shooting at #SaugusHigh, suspect is in custody and being treated at a local hospital," tweeted Los Angeles Country Sheriff Alex Villanueva.

The suspect was described as a male Asian. Local media reports said he was 15 years old.

Officials at the nearby Henry Mayo Hospital in Valencia said they were treating a total of four patients from the high school - three of whom were in critical condition.

Police spokesperson Bob Boese, speaking on the local NBC news channel, cautioned that it was not clear if all victims had been located.

"Our deputies are doing a systematic search of the campus," Boese said. He said one weapon had so far been recovered.

The Santa Clarita Valley Sheriff's department had earlier tweeted that it believed there was only one suspect involved.

A White House spokesman said President Donald Trump is "monitoring the ongoing reports."

'Always been worried'

Aerial video from the local NBC affiliate showed students with hands raised being escorted by officers from the building, and led to a nearby church, as others stood outside the perimeter on their cell phones.

Denzel Abesamis, a student at the school, told the Los Angeles Times he was arriving at campus when he saw classmates fleeing the scene.

He called a friend who was hiding in a classroom with five other students.

"I've always been worried about something like this would happen since it was my actual school," he said, adding that the school had been locked down due to reports of a threat a few years earlier.

"Saugus has emphasised the importance of making sure to always be cautious of anything that may happen like an active shooter," he said.

At least three ambulances were at the school campus, opposite a residential area, along with dozens of squad cars.

Agents from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives were among those responding.

The Santa Clarita Valley Sheriff's department had warned residents to stay clear of the area.

California senator Kamala Harris, a Democratic candidate for the 2020 election, said she was "heartbroken and praying for Santa Clarita".

"Our children and communities are being terrorised. We can't accept this," she tweeted.