Iran's President has announced the discovery of a new oil field with more than 50 billion barrels of crude — a find that could boost the country's proven reserves by a third as it struggles to sell energy amid US sanctions.

Key points: The 80-metre-deep reservoir stretches nearly 200 kilometres

The 80-metre-deep reservoir stretches nearly 200 kilometres The find would add about 34 per cent to Iran's current proven reserves

The find would add about 34 per cent to Iran's current proven reserves Iran currently has the world's fourth-largest proven deposits, but the new reserves would lift Iran to third place

The announcement by Hassan Rouhani comes as Iran faces crushing American sanctions after the US pulled out of its nuclear deal with world powers last year.

The vast field in the southwestern province of Khuzestan holds an estimated 53 billion barrels of crude, Mr Rouhani said in a speech from the central city of Yazd.

"This is a small gift by the Government to the people of Iran," he said.

"We announce to America today that we are a rich nation, and despite your enmity and cruel sanctions, Iranian oil industry workers and engineers discovered this great oil field."

The 80-metre-deep reservoir stretches nearly 200 kilometres from Khuzestan's border with Iraq to the city of Omidiyeh, he said.

The find would add about 34 per cent to Iran's current proven reserves, estimated by energy giant BP to be 155.6 billion barrels.

US sanctions hinder oil exports

Iran currently has the world's fourth-largest proven deposits of crude oil. ( Reuters: Essam Al-Sudani )

Oil reserves refer to crude that is economically feasible to extract. Figures can vary wildly by country due to differing standards, though it remains a yardstick of comparison among oil-producing nations.

Iran currently has the world's fourth-largest proven deposits of crude oil and the world's second-largest deposits of natural gas. It shares a massive offshore field in the Persian Gulf with Qatar.

The new reserves, if proven, would lift Iran to third place, just before regional arch-rival Saudi Arabia.

But it remained to be seen how much the country could benefit from the new field.

Any company or government that buys Iran's oil faces harsh US sanctions, the threat of which also stopped billions of dollars in business deals and sharply depreciated Iran's currency, the rial.

Iran had since gone beyond the deal's stockpile and enrichment limits, and started using advanced centrifuges barred by the deal. It had also begun injecting uranium gas into centrifuges at an underground facility.

The collapse of the nuclear deal coincided with a tense summer of mysterious attacks on oil tankers and Saudi oil facilities that the US blamed on Iran.

Tehran denied the allegations, though it did seize oil tankers and shot down a US military surveillance drone.

ABC/wires