Seven U.S. funeral home directors linked to a scheme to plunder corpses and sell the body parts for transplants pleaded guilty to undisclosed charges and have agreed to co-operate with investigators.

The unidentified directors secretly pleaded guilty to charges related to what investigators say was a plot to harvest bone and tissue and sell it to biomedical supply companies, Brooklyn District Attorney Charles Hynes said Wednesday in New York.

"It is clear that many more funeral home directors were involved in this enterprise," Hynes said at a news conference.

The seven entered their pleas in closed courtrooms and their names were withheld.

Defence attorneys said that among those co-operating was the director of a funeral home that took parts from the body of Masterpiece Theatre host Alistair Cooke, who died in 2004.

The four original defendants in the case pleaded not guilty on Wednesday to enterprise corruption, body stealing and other charges in the new indictment.

If convicted, they face up to 25 years in prison. All remain free on bail.

Accused allegedly made millions in scheme

Prosecutors allege Michael Mastromarino, a former oral surgeon, and three other men secretly removed skin, bone and other parts from up to 1,000 bodies from funeral homes, without the permission of families.

He allegedly made millions of dollars by selling the stolen tissue to biomedical companies that supply material for common procedures including dental implants and hip replacements.

They were charged in February with counts including body stealing, unlawful dissection and forgery in a case a district attorney called "something out of a cheap horror movie."

All the defendants pleaded not guilty before being released on bail.