Russia started fuel exports to Iran by rail this year for the first time after traditional buyers shunned trade with Moscow, according to three industry sources and exports data.

Russia and Iran, both under Western sanctions, are forging closer ties in order to support their economies and to undermine Western sanctions, which both Moscow and Tehran cast as unjustified.

Western sanctions on Russian oil products over what Moscow calls its "special military operation" in Ukraine have reshaped global fuel markets with tankers taking longer routes and suppliers choosing exotic destinations and ways of transportation.

Iran has been under Western sanctions for years with limited access to global markets.

The oil ministries of Russia and Iran did not reply to requests for comment.

Last autumn Russia's Deputy Prime Minister Alexander Novak announced the start of swap supplies of oil products with Iran, but actual shipments only started this year, Reuters sources said.

In February and March Russia supplied up to 30,000 tons of gasoline and diesel to Iran, two sources familiar with the export data told Reuters.

A third source confirmed the trade but was not able to confirm the volumes.

All the volumes were supplied by rail from Russia via Kazakhstan and Turkmenistan. One of the sources said that some gasoline cargoes were sent on from Iran to neighboring states, including Iraq, by truck.

Iran is an oil producer and has its own refineries, but recently its consumption had exceeded domestic fuel production, especially in its northern provinces, a trader in Central Asian oil products market said.

Russia had supplied small volumes of fuel to Iran by tanker via the Caspian Sea, as was the case in 2018, two traders familiar with the matter said.

Russian oil companies are currently interested in exporting diesel and gasoline to Iran by rail as exports by sea face high freight rates and a price cap imposed by the G-7 countries.

However the rail exports face bottlenecks along the route, the sources said.

"We expect fuel supplies to Iran to rise this year, but we already see several issues with logistics due to rail congestion. That may keep exports from booming," one of the sources familiar with supplies to Iran said.