MOSCOW, Sept 16 (Reuters) - Russia on Thursday threatened U.S. social media firms with sizeable fines if they fail to delete content Moscow deems illegal and demanded Apple and Google stop allegedly meddling in its internal affairs on the eve of parliamentary elections. Internet services are coming under increasing pressure ahead of Russia"s Sept. 17-19 parliamentary vote. Russia"s authorities have said foreign firms are hampering their efforts to block virtual private networks (VPNs) and online resources linked to jailed Kremlin critic Alexei Navalny. read more Now, a long-running dispute over banned content looks set to move up a gear after Vadim Subbotin, deputy head of state communications regulator Roskomnadzor, signalled sizeable fines were possible. "We will now consider applying turnover fines on those companies that systematically violate Roskomnadzor"s demands," Interfax news agency cited Subbotin as saying. MOSCOW, Sept 16 (Reuters) - Russia on Thursday threatened U.S. social media firms with sizeable fines if they fail to delete content Moscow deems illegal and demanded Apple and Google stop allegedly meddling in its internal affairs on the eve of parliamentary elections. Internet services are coming under increasing pressure ahead of Russia"s Sept. 17-19 parliamentary vote. Russia"s authorities have said foreign firms are hampering their efforts to block virtual private networks (VPNs) and online resources linked to jailed Kremlin critic Alexei Navalny. read more Now, a long-running dispute over banned content looks set to move up a gear after Vadim Subbotin, deputy head of state communications regulator Roskomnadzor, signalled sizeable fines were possible. "We will now consider applying turnover fines on those companies that systematically violate Roskomnadzor"s demands," Interfax news agency cited Subbotin as saying. While Russia has already levied several small fines on foreign tech companies, imposing penalties based on their turnover suggests potentially much bigger sums. Subbotin named Facebook (FB.O), Twitter (TWTR.N) and Alphabet"s (GOOGL.O) Google as some of the firms at risk. Roskomnadzor now has "substantial" tools capable of enforcing the requirements of Russian law, he said, without giving further details. Roskomnadzor has successfully slowed the speed of Twitter since March, impeded some VPN providers from operating, and last week blocked major domain name system (DNS) services for several hours. read more "Now, Russia is way ahead of China in terms of blocking capabilities," IT expert Mikhail Klimarev told Reuters. Earlier on Thursday, lawmaker Andrei Klimov said Russian prosecutors had made official approaches to Apple (AAPL.O) CEO Tim Cook and Google CEO Sundar Pichai on Sept. 9, telling them to stop breaking Russian law by continuing to allow people to access Navalny"s banned tactical voting app on their companies" stores. "(Apple and Google"s) actions during Russian elections are seen as illegal and directly linked to interference in Russia"s purely domestic affairs," Interfax cited Klimov as saying. Apple and Google did not immediately respond to requests for comment. Apple"s AppStore suffered an outage earlier this week and GlobalCheck, a group that monitors websites" accessibility in Russia, late on Wednesday said telecoms operators had started blocking access to Google Docs.