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
The global oil market had already factored in Iranian oil exports falling to zero under the Trump administration's economic pressure campaign against Tehran.
The White House announced it was calling an end to six-month waivers that had exempted several countries from unilateral US sanctions on Iranian oil exports
The US has made it clear that any country that continues to do business with Iran will be blocked from accessing the American banking and financial system