PRINCETON, New Jersey (AP) - A phoned-in bomb threat forced Princeton University to shutter its campus for several hours on Tuesday, but it later reopened after an extensive search by law enforcement officials turned up no explosives.

The school received the threat against multiple buildings around 9am and decided to evacuate just before 10.30am, Princeton spokesman Martin Mbugua said, noting that officials at the elite Ivy League university in New Jersey took that time to assess the threat.

Nearly 7,000 students, faculty and university workers were evacuated from the campus and off-campus buildings. Faculty and staff were sent home, while students were told to go to nearby public places.

The evacuation order then remained in place for nearly eight hours as the university's Public Safety department worked with multiple law enforcement agencies to sweep numerous campus buildings with bomb-sniffing dogs.

Princeton issued a statement saying the bomb threat was one of a number of threats around the country on Tuesday. But a university spokesman said he was unaware of anything linking them.

No classes were in session on Tuesday, though some summer programmes were.

Mr Brandon Blau, 20, a chemistry major from Long Island, New York, was in a lab working on research when the threat was announced. By the afternoon, he and fellow chemistry students were hanging out on a street near campus, checking a laptop for updates on when they could return.

"You want to be safe about it, but we also want to get back on campus," Mr Blau said. He and his friends are doing research this summer.