Ukrainian police on Thursday arrested five people thought to be the brains behind a scam using the Zeus Trojan to siphon money from small businesses in the US

The operation is part of an ongoing effort to take down a criminal empire that stole £45 million from victims' bank accounts over the past few years. Many of those hit were small businesses or local organisations that ended up having to absorb the costs of the fraud.

Ukraine's national police force, the SBU, made the arrests as part of a joint effort with the US Federal Bureau of Investigation, police in the Netherlands and the UK's Metropolitan Police Service. Those detained are "key subjects responsible for this overarching scheme," the FBI said in a statement.

Ukrainian SBU agents also executed eight search warrants in an operation that was manned by about 50 police officers.

Law enforcement officials believe they arrested a "top-tier group" of organizers of the wide-spread operation, FBI officials said at a press conference Friday. The operation used an estimated 3,500 money mules, people who transferred money back to the organisers, FBI officials said.

"We believe we've dismantled the operation" with the recent arrests, said Gordon Snow, assistant director with the FBI's Cyber Division.

The Ukraine arrests are "far more significant" than the earlier actions in the UK and the US, said Gary Warner, director of research in computer forensics with the University of Alabama at Birmingham. "These are five ringleaders," he said. "All roads lead back to these five guys. The people that were arrested in the UK and the people that were arrested in the US... all were piling their money back to these guys in Ukraine."

Zeus is considered to be the most lucrative piece of malware ever created, and it took a network of operatives to manage the operation. On Tuesday, the Metropolitan Police Service in London arrested 20 people, and that was followed up by more arrests in the US on Thursday. Both operations dismantled the "money mule' system used to move funds overseas.

US officials have charged 92 people with crimes related to the operation and arrested 39, the FBI said. The operation targeted small businesses and other organizations in the US, with the criminals often focusing on treasurers or chief financial officers of the organisations, the FBI said.

The operation, named Trident Breach, was launched in May 2009 after FBI agents in Omaha, Nebraska, began investigating a computer fraud case that involved 46 unauthorised payments made to different bank accounts across the country. This type of fraud is the trademark of the Zeus network. In a typical Zeus theft, the criminals hack into the victim's online bank account and then move money out using the banking system's automated clearing house (ACH) money transfer system.

"The cyber thieves targeted small- to medium-sized companies, municipalities, churches, and individuals, infecting their computers using a version of the Zeus Botnet," the FBI said Friday in a press release. "The malware captured passwords, account numbers, and other data used to log into online banking accounts."

According to the FBI, the scammers tried to steal £190 million in total, and actually managed to move £45 million offshore from the US There were about 390 victims in the US, the FBI said.

Zeus' main software developer, who once went by the name A-Z, is thought to live in St Petersburg, according to Don Jackson, a researcher with SecureWorks, one of the companies that has been tracking Zeus for years.

To complicate matters, Zeus is not run by a single gang. There are perhaps five to 10 Zeus gangs that operate at the highest level, Jackson said. These are the crooks who get access to the best code, who have the most up-to-date attacks, and who make the most money.

However the Zeus code is also freely sold on the black market, and there are many others who also make their own use of the malware.

Zeus is continually updated, with developers often adding features at the request of users of the malware, FBI officials said.