It took more than a letter from Syrian President Bashar al-Assad to convince his Russian counterpart Vladimir Putin to send its air force to fight the Islamic State (Isis) in the country.

Russia now reportedly enjoys an exclusive oil and gas exploration access in Syria. According to a report in WorldNetDaily (WND), the Syrian regime has reached an understanding to give an "unrestricted" access to the Russians to explore oil and gas in the country.

"The silent agreement extends beyond the multi-million-dollar deal Syria signed in 2013 with Russian company Soyuzneftegaz for exclusive offshore drilling, development and production pertaining to a 2,190 square kilometre bloc of Syria's territorial waters,"  Jerusalem bureau chief of WND, Aaron Klein, revealed in a recent report.

The agreement gives Russians access to a 2,190 sq km bloc of Syria's territorial waters -- an area which according to an assessment by US Geological Survey has 1.7 billion barrels of oil and 122 trillion cubic feet of natural gas.

In the meantime, it has emerged that the Russian oil and gas company SoyuzNefteGaz has begun oil prospecting operations in Latakia, a source told al-Araby al-Jadeed's Arabic service.

Russia has launched a massive military operation in Syria against several rebel groups, including the Islamic State (Isis). Russian air force in recent days have carried out dozens of bombings on rebel targets in Syria.

Syria said that Russia was invited by Assad, who wrote a letter to Putin requesting him to send the Russian air force to fight terrorism.