Iran has barred the mayor of Tehran from traveling to the United States for a ceremony honoring his city's advances in public transport, media reported on Saturday.
Mohammad-Baqer Qalibaf, a losing candidate in the 2005 presidential election that brought Mahmoud Ahmadinejad to power, is seen by some Iranians as a possible future presidential contender.
Both conservatives, the two have been at odds over funding Tehran's extensive underground railway network which the municipality and national parliament say is owned $1 billion from state coffers.
Qalibaf had been invited to Washington by a non-governmental organization, the Institute for Transportation and Development Policy, for a ceremony on January 24 where Tehran is one of five cities up for the "2011 Sustainable Transport Award."
Iran has had no diplomatic ties with the United States since the 1979 Islamic Revolution overthrew the U.S.-backed Shah.
Tehran-e Emrouz, a daily newspaper close to the mayor, said the Foreign Ministry had denied Qalibaf a visa.
"It is not clear why the trip ... which was in line with introducing national achievements and proving the efficiency of Iranian management, was treated in such an unkind manner and has been the victim of political considerations," it wrote.
The Foreign Ministry was not immediately available for comment.
Some two million Iranians use the Metro every day although many complain of chronic traffic congestion and pollution.
The Metro's management expects the number of Metro users to rise to 2.5 million in the next two months due to a seven-fold increase in gasoline prices, the result of Ahmadinejad's bold push to slash state subsidies.
Parliament has approved $1 billion in government investment to expand the Metro, which already has four lines criss-crossing Tehran, but the government has yet to disburse the funds.
Ahmadinejad, who was Tehran's mayor himself before becoming the surprise presidential election winner in 2005, wants five million people to permanently quit the city to relieve transport and environmental strains and reduce the potential carnage in the event of a major earthquake, which seismologists say could strike at any time.
Qalibaf played down any disappointment about not being able to travel to Washington.
"It's not important whether I go or not to this ceremony, what is important is the work that has been done that means we can now compete with the big and industrial cities of the world," he was quoted as saying by the semi-official Mehr news agency.
The other cities nominated for the transport prize are: Guangzhou, China; Leon, Guanajuato, Mexico; Lima, Peru; and Nantes, France.