Iran discovers new oil field containing 53bn barrels of crude
The field covers 2,400 square kilometres (926 square miles) and is located in Iran's southwestern province of Khuzestan, Rouhani said in a speech aired on state TV.
"This is a small gift by the government to the people of Iran," he said.
The 80-metre deep field stretches nearly 200 kilometres from Khuzestan's border with Iraq to the city of Omidiyeh, Rouhani added.
The find would add around 34 percent to the OPEC member's current proven reserves, estimated by energy giant BP at 155.6 billion barrels of crude oil.
Iran is a founding member of the Organisation of the Petroleum Exporting Countries and sits on what were already the world's fourth-biggest oil reserves and second-largest gas reserves.
But it has faced a major obstacle in selling its oil since US President Donald Trump withdrew from a landmark 2015 nuclear deal and reimposed unilateral sanctions on Iran, and threatens to punish other countries that buy Iranian oil.
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The remaining parties to the accord - Britain, China, France, Germany and Russia - have worked to save it by avoiding US sanctions, but their efforts have so far borne little fruit.
Meanwhile Iran is taking steps to fully withdraw from the accord itself, in order to ramp up uranium enrichment for nuclear power, which could help develop nuclear warheads.
Tehran emphasises the measures it has taken are swiftly reversible if the remaining parties to the deal - Britain, China, France, Germany and Russia - find a way to get around US sanctions.
On July 1, Iran said it had increased its stockpile of enriched uranium to beyond a 300-kilogramme maximum set by the deal, and a week later, it announced it had exceeded the enrichment cap.
The third move had it firing up advanced centrifuges on September 7 to enrich uranium faster and to higher levels.