The Russian nuclear equipment and services monopoly said last week it sent its Iranian partners a timeframe for settling an outstanding debt for the construction of the Bushehr NPP.
Atomstroyexport, the contractor for the NPP project, said Tehran has not made payments for the $1 billion NPP project in over a month.
"Our federal agency - Rosatom - is dealing with it [Bushehr project]. They are doing calculations together with the Iranian side. It is a technical matter," Sergei Lavrov said.
"I have nothing to add," he said.
Tehran said earlier it would be ready to resolve any problems over its purported failure to pay Russia for NPP construction within 10 days.
"Iran transferred all the necessary payments to the Russian company Atomstroyexport for the construction of the Bushehr nuclear power plant on time, in conformity with the current contract," said Mohammad Saidi, deputy director of Iran's Atomic Energy Agency.
He said the construction of the Bushehr NPP is proceeding according to plan, and that preparations are now under way to install equipment necessary to put the facility into operation.
"The problem is the Russian company's shortage of funding to import [this] equipment to Iran. Atomstroyexport has asked us to help resolve this problem," Saidi said, adding that the Iranian side was ready to bear a part of the costs involved to ensure that the necessary equipment is delivered to Iran as soon as possible.
Atomstroyexport said the lack of funding could slow the work on the project, adding that third countries are involved in equipment deliveries.
It said a meeting was held with chief executives of all sub-contractors working on the NPP to develop a common negotiating position.
Atomstroyexport said over 2,000 specialists from Russia, Ukraine, and other countries are currently working at Bushehr, while funding shortages prevent the hiring of more personnel and could delay the NPP's launch according to schedule.
The company said it forwarded to the Iranian side all the necessary documents to coordinate settlement terms on Bushehr's construction.
"We are very concerned and are awaiting proposals from the Iranian side," a spokesman for the company said.
But it said earlier that suspending or freezing the construction of the Bushehr nuclear power plant is out of the question.
Bushehr NPP "is the most important aspect of Russian-Iranian cooperation. Therefore, suspending or freezing its construction is out of the question," Vladimir Pavlov, the head of Atomstroyexport's managing body for the construction of the Bushehr NPP, said.
But Pavlov reconfirmed that Iran has not made payments to Russia for about a month under the existing contract, signed in 1995.
Pavlov said the construction of the Bushehr NPP could take more time than expected.
But Mohammad Saidi said there had been no delays in payments to Russia for the construction of the plant.
He said payments were being made under the contract and on schedule.
The Bushehr facility has been a source of international dispute, with the United States and other Western countries, raising concerns that Iran could use the project as part of a covert weapons program. Iran has consistently denied that its nuclear program has military goals.
The UN Security Council adopted a resolution in December imposing sanctions against Iran.
Russia, a key economic partner of Iran, has consistently supported the Islamic Republic's right to nuclear power under the Nuclear Non-Proliferation Treaty (NPT), and has resisted the imposition of harsh sanctions.