The US city of Los Angeles ordered all public schools closed Tuesday due to an unspecified electronic threat targeting the school system and its 640,000 students, officials said.

The US city of Los Angeles ordered all public schools closed Tuesday due to an unspecified electronic threat targeting the school system and its 640,000 students, officials said.

Ramon Cortines, the superintendent of Los Angeles schools, said he ordered the shutdown after police alerted him to a threat "made to not just one school, but many schools in this school district."

The Los Angeles unified school district tweeted: "LAUSD schools are closed today due to credible threat."

Police and FBI agents were called in to help search the more than 1,000 schools in the district, Cortines told a news conference, adding that he expected the operation to be completed by the end of the day.

The superintendant said the extraordinary measure was triggered in part by the December 2 attacks that killed 14 people in nearby San Bernardino.

"I think it is important that I take the precaution based on what has happened recently and what has happened in the past," Cortines said.

The chief of the Los Angeles school police department, Steven Zipperman, also stressed the decision was an extreme precautionary measure.

"Earlier this morning we did receive an electronic threat that mentions the safety of our schools," he told the news conference.

"In an abundance of caution, as the superintendent has indicated, we have chosen to close our schools today until we can be absolutely sure that our campuses are safe."

The news conference was held shortly after 7 am local time, before the start of the school day for most children.

Steve Zimmer, president of the Los Angeles school board, said those families whose children had already been dropped off had been contacted and asked to come and collect them.