By Edward McAllister

NEW YORK (Reuters) - It gives users super human strength, makes them impervious to pain and can be bought for as little as $2 across New York. It can also kill, police say.

Illegal synthetic marijuana, or "weaponised" marijuana as police are calling it, is spreading across New York, particularly the city's homeless population in search of a cheap high but who often wind up hospitalized or dead.

Little is understood about the drug, also known as "K-2" or "spice." A spike in hospitalizations this year, some deaths, and camera footage of crazed and violent users have prompted police to act.

"The synthetic marijuana issue has been one of great and growing concern here in New York," said NYPD Commissioner William Bratton at a press conference on Tuesday in which he also released monthly crime statistics. "You are going to see a lot more of it in the short term."

Synthetic marijuana refers to herbal mixtures sold in small packets that often contain shredded plants and chemicals that when smoked can cause hallucinations, paranoia and even cardiac arrest. A federal ban on compounds found in synthetic marijuana products was enacted in 2012 and it is illegal to sell it in New York State.

One video streamed at Tuesday's press conference showed a naked man, who police say was high on synthetic marijuana, ranting and smashing a hole in a wooden fence with his fist before being pepper sprayed and wrestled to the ground by a group of officers. In another video a man, also unclothed, crouched in the middle of a street, delirious, screaming at the ground.

Between April 1 and July 31, more than 1,900 people were admitted to hospitals in New York State after taking the substance, the NYPD said. One man high on the drug used his hands to stop an electric saw being used by police to open a door, Bratton said, leaving him covered in blood.

Police are building a picture of how the substance is sold and who is using.

Raids on corner stores and other outlets, which have been illegally selling synthetic marijuana in packages for $2 to $5, have helped slow use.

Police in New York and elsewhere have been issued with information about the drug and how to deal with users, including calling for extra backup.

(Reporting by Edward McAllister; Editing by Lisa Shumaker)