A small number of mainly elderly people have been scammed out of £1.2m in the past three weeks in the north east of Scotland, police have said.
'Vishing' - where criminals persuade victims to transfer money over the telephone - was used.
Police are now urging members of the public to be on their guard.
Det Insp Iain McPhail, from the economic crime and financial investigation unit, described the scams as "completely sickening."
Techniques for vishing can include:
- Information: the criminals already have your name, address, phone number, bank details - essentially the kind of information you would expect a genuine caller to have.
- Urgency: You are made to believe your money is in danger and have to act quickly - fear often leads people into acting without thinking.
- Phone spoofing: The phone number appears as if it is coming from somewhere else, so when you pick up the phone you already believe the caller because the number is convincing.
- Holding the line: In some cases, the criminals can hold your telephone line, so if you hang up and try to call another number, you are put straight back to the fraudsters.
- Atmosphere: You hear a lot of background noise so it sounds like a call centre rather than a guy in a basement - they either do have a call centre, or are playing a sound effects CD.
Det Insp McPhail said the criminal at the other end of the phone had no regard for their victim or the "devastation" their actions would cause.
He said: "In the majority of these cases, since the money has been transferred abroad, the chances of getting the money back is fairly slim.
"The majority of the victims in this phase of vishing frauds have been elderly, retired individuals."
Mr McPhail said the cases had involved callers pretending to be from a bank and who told their victims that their money was in imminent danger.
He added: "They ask their victim to move substantial amounts of money into foreign accounts for safe-keeping and request that they do not report the call for fear of ruining the investigation."
'Matter of minutes'
He explained: "Pension pots and retirement funds can be stolen within a matter of minutes, with hard-earned savings wiped out without warning.
"They prey on people's good nature and use scare tactics. It is cruel, it is callous and it is beyond comprehension.
"Let me be clear that banks will never make phone calls like this asking you to move money, in particular into a foreign account.
"If you do receive such a call, hang up and call the police immediately."