By Tom Miles

GENEVA (Reuters) - U.N. investigators looking into the alleged torture and murder of government opponents in Burundi have drawn up a list of suspects who should be prosecuted for crimes against humanity, they said on Tuesday.

The U.N. has verified 564 executions in the central African nation since April 2015 when President Pierre Nkurunziza sparked protests by saying he would seek a third term, the investigators said, calling that "clearly a conservative estimate".

The government called the report, by the U.N. Independent Investigation on Burundi (UNIIB), biased and politically motivated and denied all its allegations.

The investigators said they had received evidence of rapes, disappearances, mass arrests as well as torture and murder, and that there were probably many thousands of victims.

"UNIIB found that the large majority of victims have been identified as people who were opposed or perceived to be opposed to the third mandate of President Nkurunziza or of members of opposition parties," it said, adding:

"There are worrying signs of a personality cult being built around the president."

The list of suspects will be handed to the U.N. High Commissioner for Human Rights and be available in the event of any prosecutions.

"The charges of the UNIIB investigators are politically motivated and based on anonymous, unverified testimonies," the president's media advisor, Willy Nyamitwe, tweeted.

The government has sent the U.N. rights commissioner a 40-page rebuttal, he added.

UNIIB said a former senior army officer told investigators of the existence of lists of people to be eliminated.

Witnesses named 12 senior members of the security forces - who report directly to the heart of government - responsible for disappearances. Some of the people who said they had been tortured reported being held in secret jails including at the homes of the president and a government minister.

"DEPLORABLE"

The government denied the existence of such death-lists and said the accusations came from "those who want to sow division and the panic within the defense and security corps."

"It is deplorable that the experts believed such gratuitous and diversionist assertions," it said.

According to the UNIIB report, the bodies of some people who were summarily executed were transported across the Ruzizi river and buried in the Democratic Republic of the Congo.

It listed 17 types of torture used by the security forces, ranging from attaching weights to the testicles to forcing a victim to sit on broken glass or to stay next to the dead body of a relative.

Many women fleeing the country were subjected to sexual violence by the members of the youth wing of the ruling party, Imbonerakure, border guards and unidentified men. Women opposed to the president's third term were also subjected to extreme sexual violence, the report said.

Satellite imagery suggested the existence of mass graves, but the government did not respond an offer to investigate the sites, it said.

Burundi has set up three commissions of inquiry to look into human rights allegations, but the report accused the government of "blatantly failing" to investigate.

(Additional reporting by Clement Manirabarusha in Bujumbura; Editing by Robin Pomeroy)