Iran’s oil ministry says that the country has halted all oil sales to British and French companies.
European Union member states had already agreed to stop importing Iranian crude from 1 July.
The move is intended to put pressure on Tehran to stop enriching uranium.
Iran insists its nuclear program is peaceful and for civilian use only, but the UN’s International Atomic Energy Agency says it has information suggesting Iran has carried out tests “relevant to the development of a nuclear explosive device”.
Sunday’s statement on the oil ministry website was attributed to spokesman Ali Reza Nikzad Rahbar.
Some Iranian media outlets had recently reported that Iran had stopped oil exports to the Netherlands, Greece, France, Portugal, Spain and Italy in retaliation for the EU’s oil embargo, but this was later denied by the oil ministry.
The EU oil embargo, which was agreed upon last month, was phased so member states that were relatively dependent on Iranian crude, most notably Greece, Spain and Italy, had enough time to find alternative fuel sources.
The bloc currently buys about 20% of Iran’s oil exports, which account for a majority of government revenue.
However, Iran’s Oil Minister Rostam Qasemi claims that a cut in exports to Europe would not hurt Tehran.
Oil industry sources quoted by Reuters news agency say Iran’s top oil buyers in Europe have already started reducing purchases of Iranian crude.
Last year Iran supplied more than 700,000 barrels per day (b/d) to the EU and Turkey, but by the start of this year that had dropped to about 650,000 b/d.
France’s energy giant Total has stopped buying Iranian crude and Royal Dutch Shell, one of the biggest purchasers of Iranian oil, has cut back sharply.
Tupras of Turkey was the biggest European customer for Iranian oil in 2011, taking about 200,000 b/d, followed by Total (100,000 b/d), Shell (100,000 b/d), Hellenic of Greece (80,000 b/d) and Cepsa of Spain (70,000 b/d).