Russian news agencies quote Ivan Istomin, the head of Energoprogress, as saying the launch of the plant in southwestern Iran this autumn is unrealistic. Energoprogress is a subcontractor of the Russian state-owned firm building the plant, Atomstroiexport.
Istomin said a realistic time frame for starting the reactor is autumn 2008.
Russia has come under strong pressure from the West to abandon the project.
The United States suspects Iran will use the plant to aid in what it believes is Tehran's pursuit of nuclear weapons. Iran denies the charge, saying the plant is for peaceful civilian purposes.
Russia has said it will stick to the project but has repeatedly postponed the opening date for the plant, citing technical difficulties and a lack of payments from Tehran.
(Interfax, Reuters, AFP)