Shana agency said Oil Minister Masoud Mirkazemi attended the opening, which raises the field's total output to around 160,000 barrels per day (bpd), from first phase production of around 50,000 bpd.

Iran's total output is running at around 3.7 million bpd, according to latest data from OPEC.

The second phase of Darkhovin was developed by a consortium comprised of Italy's ENI, which signed a 1.3-billion-dollar contract, making it the largest Western firm operating in Iran, and local companies.

Most Western firms have exited Iran following sanctions imposed on Tehran over its controversial nuclear programme.

Iran is the second largest crude exporter in the Organisation of Petroleum Exporting Countries after Saudi Arabia, and it holds the world's second largest gas reserves after Russia.