Moscow warned Washington on Tuesday of "severe repercussions" on the relations between the two superpowers should Russian companies operating in Iran be affected by the new American sanctions imposed on the Islamic Republic.
"We assume that (Russian) companies which do violate the restrictions imposed by the UN Security Council (on Iran) will not be affected by the anti-Iranian American laws," the Russian Foreign Ministry said.
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About a week ago the US Senate unanimously approved on Monday a package of new economic Iran's oil sector.
The West suspects Iran is working to build a nuclear bomb and the sanctions are meant to strip Tehran of revenue by shutting down financial deals with Iran's powerful state oil and tanker enterprises. Iran has said its nuclear program is for civilian purposes.
The new sanctions build on penalties signed into law by President Barack Obama in December against foreign institutions trading with Iran's central bank. Those sanctions already have cut deeply into Iran's oil trade.
US Secretary of State Hillary Clinton said last week that Washington will not ease sanctions on Iran before a third round of talks between major powers and Iranian officials about Tehran's nuclear program.
Also on Tuesday, Russia's state atomic agency Rosatom said it is ready to help Iran build another unit at the Bushehr nuclear power plant.
Rosatom is prepared to offer Iran support on the project "if it is not banned and if it is profitable, if the project is developed," Interfax news agency quoted Rosatom deputy head Nikolai Spassky as saying.
The UN Security Council has permitted the expansion, so there is "nothing new" about it, Spassky told reporters, adding that preliminary consultations on the issue are underway.
Atomstroyexport, Bushehr's Russian contractor and an engineering company of Rosatom, earlier said the power plant would be operational by the end of 2012.
According to the contractor, Bushehr, the first nuclear power plant in Iran, successfully reached 90 percent of its nominal capacity in routine trials in early May.
Iran's Atomic Energy Organization chief Fereydoon Abbasi Davani said Saturday the country is to launch a second power unit at Bushehr plant next year with a capacity of 1,000 MW.
Construction of the Bushehr plant began in 1975 by several German companies. However, work halted when the United States imposed an embargo on hi-tech supplies to Iran after the 1979 revolution.
Russia signed a contract with Iran to complete the construction in 1998. However, the remaining construction was postponed several times due to mounting technical and financial challenges and pressure from Washington.
AFP, Reuters contributed to the report