Iran has agreed to supply Damascus with $3.6 billion in oil in exchange for the right to invest in the country, Syria's state news agency SANA said on Tuesday.

"An agreement was signed (on Monday) in Tehran... by the Iranian and Syrian central banks, granting Syria a credit line worth $3.6 billion," it reported.

The deal stipulates that Syria will pay back the cost of the oil loan "through Iranian investments of various kinds in Syria", said SANA.

It did not elaborate on what kind of investments Tehran would make.

Iran is the main regional backer of Syrian President Bashar al-Assad, whose regime has fought for more than 28 months to crush a protest movement that morphed into a bloody insurgency after the army cracked down against dissent.

Oil production in Syria has crashed over the course of the country's war, which the United Nations says has killed more than 100,000 people.

Previously a small energy exporter, Syria is now forced to import oil and by-products, Oil Minister Sleiman Abbas said in May.