"If we can't launch the station in September, then we cannot deliver the fuel according to the old timetable either."
Moscow and Tehran have been publicly arguing over what Russia says are tens of millions of dollars in delayed payments for its work on Bushehr.
Three days of talks last week between Atomstroiexport officials and an Iranian delegation led by Mohammed Saeedi, the vice president of the Atomic Energy Organization of Iran, ended without agreement.
Iran denies it has missed payments on the plant.
The United Nations' five Security Council permanent members and Germany in New York are currently considering a draft resolution to tighten sanctions against Iran because of its nuclear programme.
In December the council agreed a first set of measures penalising Iran for its refusal to suspend sensitive uranium enrichment activities, its first-ever sanctions against the Islamic republic.
Tehran has repeatedly refused to suspend enrichment, a process that several countries, principally the US, say could be diverted to make nuclear weapons.
"It is stubbornness and pride on Iran's part that has led to this impasse, and it can only lead to disaster for Iran"
Diom1982, Cork, Ireland
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Iranian television said on Sunday that Mahmoud Ahmadinejad, the country's president, wanted to defend Iran's position in person at a UN Security Council meeting to discuss the country's nuclear programme.
Tzipi Livni, Israel's foreign minister, said on Monday that UN sanctions are having an impact on Iran, and urged that they be "strengthened and extended without delay".
Livni told a meeting of the American Israel Public Affairs Committee (Aipac) during a visit to Washington on Monday that "the international community must not close its eyes" to the threat posed by Iran.
Ahmadinejad has repeatedly said the programme is for peaceful purposes only.