Iran has accused the UK, Germany and the UAE of refusing to provide fuel to Iranian passenger planes.
The allegation came days after the US enacted unilateral sanctions over Iran's nuclear programme, to penalise foreign companies trading with Tehran.
Britain said it was not aware of any refusal to refuel Iranian planes, and Germany stressed there was no ban.
Oil firms contacted by the BBC said they could not comment on individual contracts.
Tehran says its nuclear industry is for peaceful purposes but Western powers fear it is trying to develop a bomb.
"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 Iranian media.
He said the national carrier Iran Air and a private airline, Mahan Air, had both run into refuelling problems.
"Refusing to provide fuel to Iranian passenger planes by these countries is a violation of international conventions," he added.
Iranian lawmaker Heshmatollah Falahatpisheh said Iran would retaliate.
"Iran will do the same to ships and planes of those countries that cause problems for us," Iran's Isna news agency quoted him as saying.
A spokeswoman for the Abu Dhabi Airports Company (ADAC), which manages the airports in the UAE cities of Abu Dhabi and Al Ain, told Reuters news agency the company was continuing to refuel Iranian jets.
"We have contracts with Iranian passenger flights and continue to allow refuelling," she added.
But AFP news agency quoted "a source close to the aviation sector in the UAE" as saying there had been a problem with an unnamed international fuel supplier.
"A servicing company which provides fuelling at several airports around the world has refused to provide Iranian planes with fuel, including at UAE terminals," the source said on condition of anonymity.
A spokesman for the UK's Civil Aviation Authority told the BBC any move to withhold fuel would be down to individual fuel companies.
Germany's transport ministry said the refuelling of Iranian planes was not banned under EU or UN sanctions but it would not comment on whether any individual providers were refusing to fuel Iranian aircraft.
The US sanctions prohibit the sale or provision to Iran of refined petroleum products worth more than $5m (£3.3m) over a year.
Paul Reynolds, world affairs correspondent for the BBC News website, said it might be that fuel companies are worried that their sales over a year might add up to $5m, in which case they could face a possible ban on doing business in the US.
An Iranian aviation official said Iranian airliners were filling up with as much fuel as possible inside Iran.
But they were also having to refuel in countries along their route not imposing a ban, a move which the official said was doubling costs.
The new US sanctions were signed into law by President Barack Obama last week.
Last month, the UN Security Council voted to impose a fourth round of sanctions on Tehran for failing to halt its nuclear enrichment programme.