Eight bricks of military grade explosives, but not the detonators needed to set them off, have been discovered in a Manhattan cemetery, New York police said.

The bricks of powerful C4 explosive were found in a black plastic bag in the secluded New York City Marble Cemetery. One of the bricks was broken in two.

City police commissioner Raymond Kelly said the material was "very potent" and the same type used in the terrorist bombings in London in 2005.

"Inside a building, it could have caused an awful lot of damage," he said in a press conference outside the cemetery.

However there were no detonator caps and there was no immediate danger to the public, he said.

The source and purpose of the explosives remained a mystery. Kelly said that intelligence officers and the bomb squad would be involved in analyzing the material at a police firing range in the Bronx borough, north of Manhattan.

The bricks were first found in 2009 by a cemetery worker, who left them in the black bag at the back of the graveyard where they remained until they were discovered this weekend by a volunteer worker, Kelly said.

The material was "military grade" and was wrapped in protective covering. "It has yellow writing on a sort of military green material," he said.

It was "taken perhaps from a military installation years ago. We don't have the age," he said. "It looks like it's been there a significant period of time."

Police are also probing whether there was a link between the explosives and bizarre notes found nearby.

One note, written in chalk on the pavement, said: "I really hope one of you finds this."

Another message, posted in an envelope on a police car, said, "words to the effect of 'stop putting Christ on 2nd Street," Kelly said. It was signed "Jesus Christ."

"It's difficult to tell when this may have been written. What we'll do is look at cameras in the area," Kelly said.