The U.S. would act if Russia was to conclude any economic deal with Iran that flew in the face of international sanctions against Tehran, a State Department spokesperson has said.
"We are aware of the talks between Russia and Iran involving various areas of planned future economic cooperation," spokesperson Jen Psaki told journalists at a press briefing on Monday.
A day earlier, Russia's Economy Minister AlexeI Ulyukayev said he hoped a deal to supply grain and equipment to Iran in return for oil could be reached soon, Reuters reported.
Asked about the oil-for-grain deal, Psaki said the U.S. would act if it was sanctionable, according to a transcript on the U.S. State Department website.
Russia and Iran have forged closer ties this year, with both countries facing increasing isolation from the West. While Moscow has been targeted by U.S. and EU sanctions over its policy on Ukraine, Iran has faced United Nations sanctions over its uranium-enrichment program since 2006.
Russian President Vladimir Putin has met Iranian counterpart Hassan Rouhani three times since January, and last month Russia agreed a deal to build two more nuclear reactors in Iran to add to the one it has already built near Bushehr.
Russia, along with the U.S., China, France, Britain and Germany, is also engaged in negotiations regarding Iran's nuclear program that would see sanctions against Tehran relieved in return for assurances over its nuclear ambitions.
A November deadline to conclude talks between the six world powers and Iran came and went to no avail, and a new deadline to end the standoff has been set for July 1, 2015.