Five people have been killed in a shooting at an Islamic centre in San Diego, including two suspects, as police say they are treating the incident as a possible hate crime.

Authorities say two teenage suspects opened fire at the Islamic centre in San Diego on Monday, killing three men before killing themselves.

A security guard was among those killed in the mosque, said San Diego Police Chief Scott Wahl.

"What we know at this time is that there are three deceased adult males," San Diego Police Chief Scott Wahl said, referring to the three victims, adding that the two alleged shooters had also been killed.

Wahl, speaking at a news conference after authorities had earlier declared the threat had been "neutralised," said one of the deceased was a security guard who worked at the centre.

"Because of the Islamic centre location, we are considering this a hate crime until it's not, and at this point we're going to work closely with the FBI to make sure that we are matching all the resources that we need for this investigation," he added.

California Governor Gavin Newsom said he is aware of the shooting at the mosque and is receiving updates from law enforcement as emergency personnel respond to the scene.

“Terrible situation”

The Islamic Centre in Clairemont, a residential and commercial district of San Diego, is the largest mosque in San Diego County, according to its website.

President Donald Trump described the shooting incident as a “terrible situation."

“They're giving a briefing on it ...and it's a terrible situation. I've been given some early updates, but we're going to be going back and looking at it very strongly," Trump told reporters at the White House.

Meanwhile, Islamic Centre of San Diego Director Taha Hassane told reporters that it is "extremely outrageous" to target a place of worship.

"We have never experienced a tragedy like this before," Hassane said.

The Council on American-Islamic Relations (CAIR) in San Diego condemned the attack.

"May Allah grant him the highest rank of Paradise and protect everyone from harm. No one should ever fear for their safety while attending prayers or studying at an elementary school," CAIR said on X.

The Islamic Centre serves a congregation of more than 5,000 members, according to its website.

Its campus houses the Al Rashid School, which offers religious and language instruction.

The centre says its mission is to support the community while working with the wider public to aid the less fortunate, promote education, and contribute to America’s betterment.