Los Angeles city officials on Tuesday defended their decision to shut down all public schools following an emailed threat, as authorities in New York dismissed a similar message as a hoax.

Ramon Cortines, the superintendent of Los Angeles schools -- the second-largest education district in the United States with 640,000 students -- said the measure was ordered as a precaution, triggered in part by the December 2 attacks in nearby San Bernardino that left 14 people dead.

Cortines told a news conference that police alerted him late Monday to an email sent to several members of the school board, threatening attacks involving backpacks and "other packages."

"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," said the school district's police chief, Steven Zipperman.

California Congressman Brad Sherman told CNN that the email had come from an IP address overseas, from a person claiming to be a "extremist Muslim" who said the attacks would involve nerve gas.

The message also said the person boasted of having "32 accomplices... all ready to take action today," Sherman said, however cautioning that some of the claims lacked credibility.

In New York, however, officials said a similar threat to schools had been received and was being treated as a hoax.

"There is no credible threat to our children. We are convinced that our schools are safe," Mayor Bill de Blasio told a press conference, adding that the threat was "so generic and outlandish" that it could not be taken seriously.

"The immediate assessment by the intelligence division, and again, in consultation with the FBI, is that there was nothing credible about the threat," he said.

In Los Angeles, police and FBI agents were called in to help search the more than 900 schools in the district, Cortines said, adding that he expected the operation to be completed by the end of the day.

New York police commissioner Bill Bratton said Los Angeles school officials had over-reacted.

"We cannot allow ourselves to raise levels of fear," Bratton told reporters.

"Certainly levels of awareness, but it is not a credible threat and not one that requires any action on our part similar to what my understanding is the school system in Los Angeles took."

Bratton, who previously served as Los Angeles police chief from 2002 to 2009, said the person behind the threat may have been inspired by the television series "Homeland".

- 'Irresponsible' to criticize -

Los Angeles police chief Charlie Beck hit out at criticism that the shutdown was unnecessary, insisting it was justified with the children's safety paramount.

"It is irresponsible based on facts that have yet to be determined to criticize that decision at this point," Beck told reporters.

Authorities said they had ordered all public transport in Los Angeles to be free of charge for students on Tuesday to enable them to get home or move about the sprawling West Coast city.

Private schools in the district remained open since the threat was only directed at public schools.

The Los Angeles Unified School District comprises more than 21,000 buildings spread over 720 square miles (1,865 square kilometers) within Los Angeles and nearby communities.

California has been stunned by the massacre in San Bernardino, about an hour east of Los Angeles, carried out by US-born Syed Farook and his Pakistani wife Tashfeen Malik.

Authorities believe the Muslim couple had been radicalized for some time and may have been inspired by the extremist Islamic State group.

The couple were killed in a shootout with police following the attack, which the Federal Bureau of Investigation is treating as an act of terrorism.

According to a new Wall Street Journal/NBC News poll released on Monday, national security has become the top issue of concern for Americans in the wake of the San Bernardino assault and the recent deadly attacks in Paris.

The poll said 40 percent of respondents feel national security and terrorism should be the government's top priority and more than 60 percent say those concerns should be in the top two.

"For most of 2015, the country's mood, and thus the presidential election, was defined by anger and the unevenness of the economic recovery," said Democratic pollster Fred Yang, who conducted the survey with Republican pollster Bill McInturff.

"Now that has abruptly changed to fear."