Scotland Yard has said claims by a witness that a “VIP” sex abuse ring murdered three boys are “credible and true”.

The allegations made by the witness, known by the pseudonym Nick, have triggered a murder investigation.

Police said the alleged abuse by a Westminster-based paedophile ring lasted a decade at locations across London and the Home Counties, including military premises.

Det Supt Kenny McDonald, who is leading the homicide inquiry, told a press conference on Thursday that detectives believed the witness’s claims.

The abuse was said to have taken place from 1975 to 1984, when Nick was aged between seven and 16. Some of the abuse is linked to a flat in Dolphin Square, Pimlico, London, and at times is alleged to have taken place at “parties”. Sometimes it involved groups of men and other times a lone male.

McDonald said detectives who were experienced in child abuse and homicide investigations had interviewed Nick and concluded his account of abuse and the murder of the three boys was true.

Police have yet to recover any bodies and are searching missing persons reports for leads on the identities of the victims. They appealed for witnesses to help them with their inquiries.

McDonald said: “I appeal to men who were subjected to abuse 30 years ago to come forward. We are also investigating the murder of three young boys – we are determined to find answers.”

He said people who lived at or visited Dolphin Square in Pimlico in the 1970s “will have seen or heard something that they only understand the significance of now. Today I want to appeal directly to those other young boys, now men, who were also subject to abuse at the hands of these men.

“I believe that there were other boys who were abused, or who were present while the abuse took place. I would ask you to trust me. I will support you, and do everything in my power to find those responsible and bring them to justice. I need your accounts to help me do that.”

McDonald said he could not rule out that more than one flat in Dolphin Square was used to abuse Nick.

“Nick has described how a car would be sent to collect him, and he would be driven to Dolphin Square,” he said.

“The abuse he has detailed that he was subjected to was carried out by a man on his own, a group of men or during what have been described as parties.”

The residential development near parliament has long been popular with MPs.

Inquiries are also focused on other locations in London and the south of England, including military establishments, and Scotland Yard is working closely with military police.

Scotland Yard revealed that officers had spoken to the family of Martin Allen, who disappeared in 1979 at the age of 15, but said it was not yet possible to say whether his disappearance was linked to Operation Midlands.

Similarly, Sussex police were reviewing the disappearance and murder of eight-year-old Vishal Mehotra in 1981, but the Met said a link with their investigation was unknown at this stage.

Vishal Mehrotra went missing in Putney in 1981. Photograph: Handout

Last month Vishal’s father claimed his son may have been killed by the paedophile ring.

Vishambar Mehrotra, a 69-year-old retired magistrate, recorded a male prostitute saying in a telephone call that Vishal may have been abducted and taken to the Elm Guest House in Barnes, south-west London, in 1981, the Daily Telegraph reported.

Mehrotra took the recording to the Metropolitan police at the time but told the newspaper that they refused to investigate an allegation implicating “judges and politicians”.

The skull and several rib bones of Vishal were discovered in 1982 by pigeon shooters in remote marshland at Durford Abbey farm in Rogate, close to the Hampshire-West Sussex border.

Vishal, from Putney, south-west London, disappeared while shopping with his nanny and sister on 29 July 1981.

Mehrotra told the newspaper: “I was contacted by a young man who seemed to be in his 20s. He told me he believed Vishal may have been taken by paedophiles in the Elm Guest House near Barnes Common.

“He said there were very highly placed people there. He talked about judges and politicians who were abusing little boys.”