Police in California are looking for a man who reportedly walked into a California mosque saying he had a gun and threatened to kill everyone, a spokeswoman said Friday.

The man entered the Ibrahim Khalilullah Islamic Center in Fremont, south of San Francisco, during afternoon prayer Wednesday, said mosque leaders, urging official action to track down the man.

According to witnesses, a "white male in his thirties entered the IKI Center Mosque during prayer, and shouted that he had a gun and wanted to kill everyone. When confronted, the male left," said a police spokeswoman.

"We are a little worried because of the situation that happened in Oregon," said Fareed Wardak, a member of the center's board of directors, referring to a shooting in the western US state that left three dead on Tuesday.

"We don't want the same thing to happen," he told KTVU television, in comments made before Friday's shooting at an elementary school in the eastern state of Connecticut that killed 20 children and six adults.

The man left without taking out a gun. Fremont Police Department spokeswoman Geneva Bosques told AFP: "He never displayed a gun, kept his hands in his pockets. We are following up in an attempt to look at video surveillance."

The man possibly drove a 1990s gray Toyota Camry, and officers will be carrying out extra patrols looking for anything suspicious, KTVU reported.

"We do have surveillance cameras. We'll be getting pictures out of that system," said Wardak, adding: "We're not sure what kind of person" the man was.

"It's Christmas, people are going back and forth," said Mohammed Zarabi, president of the board of directors. "Maybe he needs some money. I can't say what his intention was. I just hope it doesn't happen again."

The center has some 500 members, mostly of Afghan descent, and has been in the city for two decades.

This week's shootings in Oregon and Connecticut have revived the perennial debate about gun control in the United States, a country that regularly has to cope with such apparently random killings.

Friday's massacre at the Sandy Hook Elementary School in Newtown, Connecticut was the second deadliest US school shooting, after the 2007 campus shootings at Virginia Tech University, which left 32 dead.