The 1,500-kilometer (750-mile) project was first announced in July 2011 as Syrian rebels began stepping up the fight to topple Assad. Many analysts predicted the pipeline would remain in the planning stages because of the countless risks involved, but Iran's decision to start work even just the beginning sections is seen a public show of confidence in Assad's ability to ride out the uprising.
It also reflects Iran's wider efforts to expand natural gas and oil pipeline to Middle East and Asian markets as Western sanctions over Tehran's nuclear program cut into sales. The United States and its allies accuse Tehran of seeking to develop atomic weapons, an allegation the Iranians deny.
The semiofficial Fars news agency said Iran has already begun construction of the first phase of the project involving a 225-kilometer (140-mile) stretch at an estimated cost of $3 billion. The pipeline will carry gas from the giant South Pars field in the Persian Gulf to Syria via Iraq, whose government has close ties with Iran.
Fars said the entire project is to be completed in the second half of 2013.
Economist Saeed Leilaz, however, described it as more showmanship than a serious plan at this stage.
"Given the ongoing civil war in Syria, such a project can't be implemented now. Lack of security and political instability in Iraq and Syria doesn't allow this project to be enforced at least at this point," Leilaz said. "This is a symbolic gesture by Iran to show that it can bypass Western sanctions."
Iran already supplies gas to several other countries, including Armenia and Turkey, and has plans to build a pipeline to Pakistan. Iran has the world's second-largest natural gas reserves after Russia.