Prime Minister Anutin Charnvirakul has expressed condolences to the family of two Russian siblings killed in a high-profile murder case and apologised to Russian tourists and the Russian people over the incident.

He urged police and prosecutors to expedite the investigation into the murders of the two Russian siblings and a Thai family of three so the cases could be brought to court as quickly as possible.

He condemned the killings as a grave crime that had damaged Thailand's reputation and public confidence.

Mr Anutin described the murders as a disgrace committed by two Thai suspects, warning the crime risked undermining years of efforts to promote Thailand as a safe tourist destination and a reliable place for international investment.

It began after police arrested ex-convict Thana "Pong" Kerdthong, 43, and Thongchai "Thong" Srinil, 39, in Sa Kaeo on Friday in connection with the disappearance of Russian siblings Diana Nazimova, 22, and Roman Nazimova, 17, who had been missing since July 26.

The suspects allegedly led police to a forest burial site in Bang Lamung, where the siblings' bodies were found in a shallow grave. During the investigation, officers also discovered the bodies of a Thai family of three buried in a nearby coconut grove after Thana allegedly confessed to killing them during a robbery.

Police later arrested two additional suspects, Chatchanan Paen-aiam and Atsadang Chuwong, and are investigating whether the group may be linked to other crimes. The four suspects were taken to the Pattaya Provincial Court on Saturday for pre-trial detention, with investigators opposing bail because of the seriousness of the offences and the severe penalties involved.

They face multiple charges, including premeditated murder, concealing bodies, rape, robbery and illegal possession of firearms and explosives. The court denied bail for the suspects.

Mr Anutin cancelled his engagements in Suphan Buri on Saturday afternoon and travelled by helicopter to Chon Buri to inspect the crime scenes and monitor the investigation.

He first visited the forest in tambon Huai Yai of Bang Lamung district, where the Russian siblings' bodies were found, before travelling about two kilometres away to a coconut grove where the bodies of a Thai father, mother and daughter were discovered.

Police said the arrest of the two suspects on Friday led investigators to uncover the second murder case, with the family's bodies found buried nearby after they had been reported missing for more than a month.

After receiving briefings from police and rescue workers, Mr Anutin said the government would not support sentence reductions for perpetrators of serious crimes that undermine public confidence and Thailand's tourism image. He also ordered authorities to tighten security measures and increase patrols to prevent similar incidents.

"The killings are tantamount to killing the entire nation because they have damaged public confidence and tarnished Thailand's reputation," he said.

Mr Anutin said the crimes were the actions of individuals and did not reflect Thailand's overall level of public safety. He pledged investigators would pursue all accomplices and ensure everyone involved was brought to justice.

He also expressed sympathy to the victims' relatives, noting that the Russian siblings' mother was already in Thailand. Authorities had been instructed to provide every possible assistance, including facilitating the repatriation of the victims' bodies.

"If you ask why I came here, it is to ensure everything is carried out as quickly as possible, whether it is the investigation or returning the bodies to their homeland," Mr Anutin said.

"We must show that Thailand is deeply sorry for what happened, provide every possible assistance and restore confidence. An incident like this must never happen again."

Responding to reports that one suspect had previously served prison sentences before allegedly committing the murders, Mr Anutin said authorities would review law enforcement measures for repeat offenders.

Answers wanted: Prime Minister Anutin Charnvirakul visits Chon Buri to follow up on the murder case. Khao Chonburi Wannee