The US slapped fresh sanctions on Iran Tuesday, accusing it of creating a complex web of Russian cut-out companies and Syrian intermediaries to ship oil to Damascus, which in turn bankrolled Hezbollah and Hamas.
The US considers both Hezbollah, a powerful Lebanese militia, and Hamas, the Palestinian Islamist group that controls the Gaza Strip, to be terrorist organisations.
The US Treasury said in a statement that Tehran, "working with Russian companies, provides millions of barrels of oil to the Syrian government" of President Bashar al-Assad.
"The Assad regime, in turn, facilitates the movement of hundreds of millions of US dollars to the Islamic Revolutionary Guard Corps-Qods Force for onward transfer to Hamas and Hezbollah," it said.
The new sanctions also target a Syrian national, Mohamed Alchwicki and his Russia-based company, Global Vision Group. He is accused of playing a central role both in the transfer of oil to Syria and the funneling of money to the militant groups.
The US said his company had illegally received transfers of funds from the Iranian Central Bank via a set of complex transactions.
Intermediary firms involved in the plot to obscure the real destinations of the oil and the money included a subsidiary of the Russian Ministry of Energy, according to the statement.
Secretary of State Mike Pompeo warned on Twitter that there were "grave consequences for anyone shipping oil to Syria, or trying to evade US sanctions on the Islamic Republic's terrorist activities."
He added that Iran's supreme leader Ayatollah Ali Khamenei "should decide if spending the Iranian people's money on the Iranian people is more important than investing schemes to fund Assad, Hezbollah, Hamas and other terrorists."