Iran said on Thursday that Russia has asked for Tehran’s payment for its Bushehr nuclear power plant before the time is due, the semi-official Fars news agency reported.

In order to continue the construction of the Bushehr plant, Russia “demands for the money resources before its time is due,” Iran’s chief nuclear negotiator Ali Larijani was quoted as saying.

According to the report, Iranian Atomic Energy Organization’s vice president Mohammad Saeedi and Russian contractor Atomstroiexport’s president Sergei Shmatko started talks in Moscow on Wednesday on the dispute over Tehran’s payment, the report said.

Larijani, who is also secretary of Iran’s Supreme National Security Council (SNSC), said that no information was available for now about the talks, which were expected to continue on Thursday.

The major issue was not the money itself, but the general agreement was at the stake, he added.

Last month, Atomstroiexport warned of delaying the launch of the Bushehr plant, citing late payments and problems with equipment deliveries.

Located in southwestern Iran, Bushehr is the country’s first nuclear power station, but work at the plant has been long delayed.

Russia and Iran signed a one-billion-U.S.-dollar contract for the plant in 1995. Under an agreement reached last September, fuel deliveries to Bushehr are scheduled to start in March and the plant is due to come on stream in September this year.