Russian cyber criminals used malware planted on Android mobile devices to steal from domestic bank customers and were planning to target European lenders before their arrest, investigators and sources with knowledge of the case told Reuters.

Their campaign raised a relatively small sum by cyber-crime standards - more than 50 million roubles ($892,000) - but they had also obtained more sophisticated malicious software for a modest monthly fee to go after the clients of banks in France and possibly a range of other western nations.

Russia's relationship to cyber crime is under intense scrutiny after U.S. intelligence officials alleged that Russian hackers had tried to help Republican Donald Trump win the U.S. presidency by hacking Democratic Party servers. The Kremlin has repeatedly denied the allegation.

The gang members tricked the Russian banks' customers into downloading malware via fake mobile banking applications, as well as via pornography and e-commerce programs, according to a report compiled by cyber security firm Group-IB which investigated the attack with the Russian Interior Ministry.

The criminals - 16 suspects were arrested by Russian law enforcement authorities in November last year - infected more than a million smartphones in Russia, on average compromising 3,500 devices a day, Group-IB said.

The hackers targeted customers of state lender Sberbank, and also stole money from accounts at Alfa Bank and online payments company Qiwi, exploiting weaknesses in the companies' SMS text message transfer services, said two people with direct knowledge of the case.

Although operating only in Russia before their arrest, they had developed plans to target large European banks including French lenders Credit Agricole, BNP Paribas and Societe General, Group-IB said. A BNP Paribas spokeswoman said the bank could not confirm this information, but added that it "has a significant set of measures in place aimed at fighting cyber attacks on a daily basis". Societe General and Credit Agricole declined comment.

The gang, which was called "Cron" after the malware it used, did not steal any funds from customers of the three French banks. However, it exploited the bank service in Russia that allows users to transfer small sums to other accounts by sending an SMS message.

Having infected the users' phones, the gang sent SMS messages from those devices instructing the banks to transfer money to the hackers' own accounts.

The findings illustrate the dangers of using SMS messages for mobile banking, a method favored in emerging countries with less advanced internet infrastructure, said Lukas Stefanko, a malware researcher at cyber security firm ESET in Slovakia.

"It's becoming popular among developing nations or in the countryside where access to conventional banking is difficult for people," he said. "For them it is quick, easy and they don't need to visit a bank... But security always has to outweigh consumer convenience."