Five men in Vietnam were arrested for allegedly digging up a grave and holding its skull to ransom, authorities said.

The men, aged between 31 and 45, were detained Tuesday and accused of "encroaching on graves and extorting assets," said Hoang Quoc Van, a police spokesman in the north-eastern province of Quang Ninh.

In December, local man Nguyen Hoang Quan reported to police his mother's tomb had been broken into and her skull was missing. She died in 2007.

In the coffin was a note with a telephone number asking for 15,000 dollars to return the skull.

Ancestor worship is widespread in Vietnam, where great care is taken to make sure the departed have everything they need in the afterlife, and their remains are not disturbed.

Relatives have responsibility for maintaining graves and shrines, and violation of tombs is thought to incur the displeasure of the deceased's spirit, and to bring bad luck on their descendants.

"This is the first time such a case has happened," Van said. "The gang's actions were so inhumane, brutal, causing anger and anxiety among the public."

Police have recovered the skull. If found guilty, the gang could each receive up to 15 years for extortion and five years for digging up a grave.