A separate shipping source familiar with the movements of the Russian-flagged vessels said the ships visited Cypriot and Greek ports before delivering fuel to Syria.
The Russian defence and transport ministries did not initially respond to requests for comment. The defence ministry later said EU sanctions on fuel supplies to Syria could not be applied to the Russian air group in that country.
A spokeswoman for EU foreign affairs and security policy said the implementation of EU restrictions lay with member states. "We trust that competent authorities are complying with their obligation to ensure respect of the restrictive measures in place and to pursue any circumvention attempts," she added.
Greece's foreign ministry referred questions to the shipping ministry, which was not immediately available to comment.
The Cypriot government said its authorities had not approved the docking of any Russian tankers carrying jet fuel bound for Syria. "We would welcome any information that may be provided to us on any activity that contravenes U.N. or EU restrictive measures," the Cypriot foreign ministry added.
Syria's civil war, which began in 2011, has become a theatre for competing global powers, with Russia and Iran supporting President Bashar al-Assad, and the United States, Gulf Arab and European powers backing rebels who want to depose him.
Russia changed the course of the conflict in favour of Assad's government last year when it intervened with air strikes. Moscow says it targets only Islamic State militants and other jihadist fighters.
EU Council Regulation 1323/2014, introduced two years ago, bans any supply of jet fuel to Syria from the EU territories, whether or not the fuel originated in the European Union.
Over one two-week period in October, Russian tankers delivered 20,000 metric tonnes of jet fuel to Syria - worth around $9 million at today's world prices - via the European Union, according to the EU government intelligence source.
"The jet fuel shipments from these vessels have played a vital role in maintaining Russian air strikes in the region," said the source. "This points to a sustained Russian build-up of resources needed to support their military operation and ambitions in Syria."
Some of the shipped fuel also went to the Syrian military, helping to "keep Assad's air assets operational", the source added.
The shipping source and a third person, an intelligence consultant specializing in the Mediterranean area, also said the fuel was likely intended for Russian and Syrian military use.
