The head of Iran's Atomic Energy Organization, Gholam Reza Aghazadeh, said Russian supplier Atomstroiexport has signaled that work and delivery of fuel could be delayed again due to late Iranian payments.
But he said the real problem is that the Russian company lacks money.
Aghazadeh also said he hopes Russia is not "politicizing" the issue of the controversial Bushehr plant.
U.S. officials have long opposed the construction of the Bushehr nuclear facility.
Reports suggest that Russian officials have gone to great lengths, in talks with UN Security Council permanent members plus Germany, to resist UN sanctions that might interrupt the multibillion-dollar Bushehr project.
Those major-power talks are aimed at curbing an Iranian nuclear program that some fear is aimed at developing nuclear weapons, despite assurances from Tehran that its nuclear intentions are purely peaceful.
The Iranian Students News Agency (ISNA) today quoted Mohammad Saidi, deputy head of the Atomic Energy Organization, as saying that "if the nuclear-fuel deliveries promised to Iran are not executed by the end of March, then this matter would no longer [entail] a legal and technical [character]."
Saidi reiterated recent vows that "Iran is ready to pay the announced money to the Russian side even earlier than what we had agreed" in order to avoid delays.
(AFP, dpa, ISNA)