US agents have helped Ukrainian authorities uncover a group of computer hackers who used forged banking details to steal some $20 million from private US accounts, officials said Monday.

Four people have been detained, the former Soviet republic's security service (SBU) said, added that the group also made unauthorised transfers from Ukrainian and other countries' private bank accounts.

"Members of the group have created an extensive, well-hidden network throughout Ukraine, which was composed of more than 20 people from Ukraine and foreigners," the SBU press service said.

Officials did not say how much the group is alleged to have stolen in total, or what charges those arrested face.

According to the preliminary US law-enforcement agency data, US banks may have lost over 20 millions dollars because of the hackers.