The accord comes after Iraq gave its green light to the project in August, and experts from the three countries will now prepare a study to develop it.
Iran is already building a gas pipeline to Turkey, and the line through Iraq to Syria would branch off of that.
Visiting Iranian Oil Minister Massoud Mir Kazemi also discussed with his Syrian counterpart the possibility of Iran and Venezuela jointly building a refinery in the centre of the country, Sana said.
The possibility of such a facility, with a capacity of 140,000 barrels, was raised by Venezuelan President Hugo Chavez when he visited Syria in October.
