Judicial authorities in Barcelona ordered "three European arrest warrants issued against Ukrainian citizens considered the leaders" of a criminal organisation involved in money laundering, police said in a statement late Thursday.
"Among them was a major businessman in the gas sector in Ukraine," the statement said. Approached by AFP, a police spokeswoman refused to identify the oligarch.
But Spanish media named him as Firtash, one of Ukraine's richest men, who made a fortune importing gas to Ukraine from Russia and Central Asia via his group Rosukrenergo in collaboration with Russian gas giant Gazprom.
ABC said Firtash was accused of "close links with organised crime and having laundered 10 million euros ($10.6 million) in Spain that came from suspected criminal activities".
Firtash was arrested in 2014 in Austria before being released on bail set at 125 million euros. Austrian authorities refused a request for his extradition in April last year to the United States, where he was accused of corruption and membership of a criminal organisation.
He was suspected of bribing civil servants in India in 2006 in order to secure titanium mining licences. Spanish police, announcing the arrest warrant, also said five suspected members of the same network had been arrested Thursday in Barcelona and in the southern resort of Marbella.
A police spokeswoman told AFP they were all Spanish nationals, describing them as "second-tier collaborators" in the enterprise -- three lawyers and two bank employees who allegedly facilitated the money laundering.
In an initial operation against the network in July, 12 people were arrested including Stepan Chernovetsky, son of colourful former Kiev mayor Leonid Chernovetsky, who founded major bank Pravex.
Police said the network had laundered illicit gains from companies based mainly in Cyprus and the Virgin Islands.
"The capital had been invested in the hotel business and in real estate worth 10 million euros," the statement said.