The Russian firm leading construction of the plant in the southern city of Busheher, Atomstroiexport, has signalled that completion of the facility and delivery of fuel could be delayed again due to late Iranian payments.

"The problem of the Russian company is one of a lack of money," retorted Gholam Reza Aghazadeh, the head of Iran's atomic energy agency.

"This goes beyond our legal engagements but, as we want that the Bushehr power station is finished as quickly as possible, it is necessary to solve this problem," he told the semi-official Mehr news agency.

Under a deal reached between Tehran and Moscow last September, Russia was to deliver nuclear fuel to Iran in March, the power station would begin working in September and it would start producing energy in November.

The mutual acrimony and delays come amid pressure from the United States on Russia to cut its assistance for Iran's nuclear programme, which Washington alleges is aimed at making atomic weapons, a charge denied by Tehran.

"Today, our nuclear question has become politicised. The start-up of Bushehr has had its delays and we hope that in the current situation the Russians will not politicise this question," said Aghazadeh.

He claimed that the chief executive of the Russian company had "changed eight times" and the firm had failed to use Tehran's payments properly.

"It is for this reason that they have liquidity problems. We always paid our financial obligations in advance," he added.

Aghazadeh's comments came after talks between Atomstroiexport and Iranian officials in Moscow last week ended without agreement, with the Russian firm again accusing Tehran of failing to pay up on time.

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