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

"An agreement was signed [on Monday] in Tehran by the Iranian and Syrian central banks, granting Syria a credit line worth $3.6bn," the agency reported on Tuesday.

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 turned into a armed revolt after the military launched a violent crackdown against dissent.

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

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