(Reuters) - An annual pillow fight among freshman at the United States Military Academy turned violent and left 30 cadets injured, The New York Times reported on Friday.

The event, part of a tradition for first-year students at the service academy in West Point, New York, is intended as a way to help them blow off steam and build morale after a summer spent preparing for a gruelling programme, the newspaper reported.

But instead, the fight turned bloody after many of the cadets stuffed helmets and other hard objects into the pillow cases resulting in concussions, broken bones, and at least one cadet knocked unconscious, the paper reported, citing interviews with officials, cadets and videos posted on social media.

No cadets have been punished and all the injured, including 24 with concussion, have returned to campus, the Times said.

The brawl on Aug 20 was not confirmed to the Times until Thursday, the newspaper said.

Lieutenant Colonel (LTC) Christopher Kasker, an academy spokesman, told the Times the matter was under investigation and there were no plans to cancel the tradition.

Reuters could not reach LTC Kasker for comment on Saturday.

"West Point applauds the cadets' desire to build esprit and regrets the injuries to our cadets," LTC Kasker told the Times. "We are conducting appropriate investigations into the causes of the injuries."

He said upperclassmen supervising the freshmen had required the cadets to wear helmets, but many put the helmets in their pillow cases instead, the Times reported.

The pillow fights date back to at least 1897, the Times reported, citing a 1901 congressional inquiry on hazing. The 2013 fight was cancelled after a cadet put a lockbox in a pillow case during the 2012 event and hurt other cadets, the paper reported.