French energy company Total says it has stopped petrol deliveries to Iran, amid growing international pressure over Iran's nuclear programme.
Total confirmed the move days after the US Congress proposed unilateral sanctions that could punish companies doing business with Iran.
The sanctions still have to be signed into law by US President Barack Obama.
The US and some allies believe Iran is seeking a nuclear weapon. Tehran says its nuclear programme is peaceful.
It was not immediately clear how much petrol Total had been supplying to Iran, or exactly when deliveries were stopped.
But the Financial Times newspaper quoted traders as saying that Total had stopped supplies about a month ago.
Dependence on imports
The US sanctions aim to stop international companies from investing in Iran's oil and gas industries or supplying it with petroleum products by preventing them from trading in the US if they trade with Iran.
Such measures are not part of the most recent round of sanctions approved by the UN, since Russia and China, both investors in Iran's energy sector, objected.
Other companies have already taken similar action to Total in anticipation of the measure passed by Congress.
Spain's Repsol confirmed on Monday that it had withdrawn from a contract it won with Royal Dutch Shell to develop the South Pars gas field in southern Iran, Reuters news agency reported.
The White House says President Obama will probably sign the Congress bill into law later this week.
The American approach is designed to target a weak part of the Iranian economy, BBC world affairs analyst Paul Reynolds reports.
Despite its huge oil and gas reserves, Iran still cannot refine enough petroleum products for domestic demand and depends on imports for 30-40% of its petrol needs.
However, Iran has other investors and suppliers and the practical effects remain to be determined.
Also, the extent to which Mr Obama may use the powers contained in the sanctions proposed by Congress remains uncertain.
He has the right to waive them on a case-by-case basis and might be reluctant to act if sanctions impacted adversely on US diplomatic efforts against Iran.