OIEC's Head Roknoddin Javadi announced that the company has agreed to finish the project with an investment of over $5 billion in four years.
Javadi underlined that this will be the first time a single Iranian firm develops two entire phases in the giant gas field.
The South Pars gas field has reserves of about 14 trillion cubic meters of gas - or about eight percent of the world's reserves.
Meantime, Chief Executive of the National Iranian Oil Company (NIOC) Seyfollah Jashnsaz said that once put into operation, the two phases will produce 50 million cubic meters of gas per day for domestic use, 77,000 barrels of gas condensates for export and 1 million tons of ethane, which is used as feedstock for petrochemical plants.
Despite US pressures, some western companies, such as France's Total, are Iran's partners in developing other phases of South Pars.
Iran, which sits on the world's second largest reserves of both oil and gas, is facing US sanctions over its civilian nuclear program.
Iranian officials have dismissed US sanctions as inefficient, saying that they are finding Asian partners instead. Several Chinese and other Asian firms are negotiating or signing up to oil and gas deals.
Following US pressures on companies to stop business with Tehran, many western companies decided to do a balancing act. They tried to maintain their presence in Iran, which is rich in oil and gas, but not getting into big deals that could endanger their interests in the US.
Yet, after oil giants in the West witnessed that their absence in big deals has provided Chinese, Indian and Russian companies with excellent opportunities to sign up to an increasing number of energy projects and earn billions of dollars, many western firms are increasingly showing interest to invest or expand work in Iran.
Some European countries have also recently voiced interest in investment in Iran's energy sector after a gas deal was signed between Iran and Switzerland regardless of US sanctions.
The National Iranian Gas Export Company and Switzerland's Elektrizitaetsgesellschaft Laufenburg signed a 25-year deal in March for the delivery of 5.5 billion cubic meters of gas per year.
The biggest recent deal, worth €100m ($147m, £80m), was signed by Steiner Prematechnik Gastec, the German engineering company, this year to build equipment for three gas conversion plants in Iran.