Four buildings at Harvard University in Massachusetts have been evacuated after unconfirmed reports of explosives on the campus.

Students and staff have been asked to evacuate the four buildings which are all around the centre of the campus.

University Police said the buildings had been evacuated "out of an abundance of caution" after receiving an unconfirmed report this morning.

University and Cambridge Police are on the scene.

In a statement, the college said "Harvard's focus is on the safety of our students, faculty and staff. We will update the media when we have more information".

Harvard houses some 2,100 faculty members and more than 10,000 academic appointments, as well as about 6,700 graduate and professional students.

Update: HUPD/Cambridge Police on scene. Unconfirmed report that explosives may have been placed in 4 campus buildings http://t.co/dr5PFJr2Is

— Harvard University (@Harvard) December 16, 2013

The school, where final exams have begun, posted alerts on its emergency page and Twitter account.

One student said she was preparing to start an exam when a fire alarm went off and everyone was ordered to leave the hall.

Freshman Hamidah Mahmud said "We were about to take our science final in the Science Center. We had our pencils out, we were about to start it and right when he was about to say 'go' the fire alarm started and they evacuated everyone".

"Because the Boston bombings happened so recently, I think they are taking extra precautions. It was pretty scary".

The Harvard Crimson newspaper reported that students have been moved to Annenberg Hall, one of the college's main eateries.

The paper said that Harvard dean of undergraduate education Jay Harris told those assembled in the hall that the morning's final exams had been cancelled.

Harvard's main campus covers 209 acres in a heavily-populated area, around 5 kilometers from downtown Boston.