Iran complained on Monday that its planes had been denied fuel in Germany, Britain and the United Arab Emirates, and Washington said private firms were making the "right choices" by cutting business ties with Tehran.
In London, The Financial Times newspaper said oil major BP had stopped refueling Iranian jets. BP declined to confirm the report but said: "We fully comply with any international sanctions imposed in countries where we operate."
Pressure is mounting on Iran over its nuclear program and the United States has stepped up its push to isolate Tehran economically. On Thursday, President Barack Obama signed into law far-reaching sanctions that aim to squeeze the Islamic Republic's fuel imports and deepen its international isolation.
"Since last week, our planes have been refused fuel at airports in Britain, Germany and UAE because of the sanctions imposed by America," Mehdi Aliyari, secretary of the Iranian Airlines Union, told Iran's ISNA news agency. So far national carrier Iran Air and Mahan Airlines had been affected, he said.
A source in the UAE familiar with the issue said a private firm had refused to refuel an Iranian plane there, but the UAE had imposed no ban of its own.
"The UAE has nothing to do with it," the source said. "They (Iranian planes) are more than welcome." The source added: "It is just one company and there are other companies. There are other service providers and Iranians can seek deals with them."
A British government source said halting supplies would be a commercial decision by private firms, but Britain was not aware of any such incident there. The German Transport Ministry said there was no ban on refueling Iranian flights in Germany.
Sources in the Middle East and Europe said that if flights were indeed denied fuel, it would be a decision by private firms attempting to comply with a new U.S. sanctions law, rather than a ban imposed by the countries where the planes landed.
"The costs of doing business with Iran, a country that is shirking its international obligations across the board and engaged in illicit activity, are rising," a senior Obama administration official said on Monday.
"The international commercial sector is making the right choices. It's now time for Iran to make the right choice — to fulfil its international obligations — that remains our primary objective," the official said.
Iranian lawmaker Heshmatollah Falahatpisheh said "Iran will do the same to ships and planes of those countries that cause problems for us."
Western powers believe Iran is trying to build bombs under cover of a civilian nuclear program. Tehran says the program is only for electricity generation and medical purposes.
Sanctions appear to be having an impact. Over the past weeks a number of countries and firms have cut back on imports of Iranian crude oil. Other companies have stopped providing Iran with refined petroleum.
The UAE took steps last week to tighten its crucial role as a trading and financial lifeline for Iran. The UAE Central Bank asked financial institutions to freeze the accounts of 40 entities and an individual blacklisted by the United Nations for assisting Iran's nuclear or missile programs.
The U.S. measures go far beyond U.N. sanctions imposed last month, by targeting firms that sell refined oil products such as gasoline to Iran, which despite being a major crude exporter imports oil products because of a lack of refining capacity.
However, Gala Riani at IHS Global Insight said preventing the fuelling of Iranian flights seemed a "very strict reading" of the new U.S. law, which is more directed at trade in fuel.
U.S. officials have not made clear whether they believe the new law bars firms from refueling Iranian aircraft. The law bans the sale or provision to Iran of refined petroleum products worth more than $5 million a year.