Police and emergency vehicles work at the scene of a shooting at a backyard party in California. AP

Ten people were shot and four of them were killed Sunday at a party in Fresno when suspects sneaked into the backyard and fired into the crowd, police said.

The shooting took place about 6pm on the city’s southeast side while people were gathered to watch a football game, Fresno Police Lt. Bill Dooley said.

Deputy Chief Michael Reid told the Fresno Bee and the KSEE/KGPE TV stations that a total of 10 people were shot, with three people being found dead in the backyard. A fourth victim died at the hospital.

Fresno Police Lt. Bill Dooley speaks to reporters at the scene of a shooting at a backyard party in California. AP

Six others are expected to survive and are recovering at the hospital, police said.

"What we do know is that this was a gathering, a family and friend gathering in the backyard,” Dooley said. "Everyone was watching football this evening when unknown suspects approached the residence, snuck into the backyard and opened fire.”

Others were taken to Community Regional Medical Center in critical condition, and some are in critical but stable condition, the TV stations reported.

No suspect is in custody.

Agents from the US Bureau of Alcohol, Tobacco, Firearms and Explosives field office in San Francisco are responding to the shooting, the Bee reported.

It was at least the second fatal gun attack Sunday in southeast Fresno, the Bee reported. A man in his 20s was shot to death early Sunday at a home in another part of the city.

The suspect fled the scene and police were combing the neighborhood for witnesses and possible security camera footage.

Police did not release the number of dead at a news conference late on Sunday, but said there were multiple deaths among the nine known to have been shot in the city, about 200 miles (322 km) north of Los Angeles.

Others at the party may have driven themselves to hospital, police said, adding that some victims were taken to a nearby community medical center with gunshot wounds.

A police spokesman was not immediately available to Reuters for further comment.