NEW YORK (AFP) - US authorities on Tuesday briefly evacuated nine schools in New Jersey after receiving anonymous bomb and shooting threats, authorities said.

The schools affected were in Bergen County in the northeastern part of the state, across the Hudson River from Manhattan. Students were evacuated in the morning but were authorised to return to class by early afternoon.

Police in the town of Clifton said "numerous school districts in the area" had received a bomb threat by voice mail early on Tuesday.

Police were dispatched to Clifton High School but found "no credibility to these threats" although "precautions are still being taken throughout the district in response to the situation," it said in a statement on Facebook.

The message was apparently recorded overnight and "indicated a non-specific threat to the school district involving the placement of a bomb in one of the schools, as well as a secondary threat of a 'mass shooting,'" police wrote.

The schools evacuated are located in Bergenfield, Englewood, Fair Lawn, Garfield, Hackensack, Leonia, Tenafly and Teaneck, said the spokesman for Bergen County Sheriff Michael Saudino.

In November 2014, a series of bomb threats forced the evacuation of seven schools in the same county. Investigators found nothing.

Last month, Los Angeles shut down the second-largest school district in the United States, keeping 640,000 students at home following an e-mailed threat that was later deemed not credible.

The drastic decision was taken after a radicalised Muslim couple shot dead 14 people in nearby San Bernardino on Dec 2.