On Tuesday, the US Treasury Department announced new sanctions against several companies in both Iran and Russia, accusing them of having played a role in shipping oil to Syria. Secretary of State Mike Pompeo declared them an exploit of the "international financial system."
It's not clear if this is meant as a sanction against Iran or a sanction against Syria, and the Treasury Department's statement offered some conflicting narratives on it, on the one hand accusing the deals of helping Syrian President Bashar al-Assad stay in power, but also claiming it was providing money for Iran to "fund terrorist activity."
With the Syrian war having left that nation's oil and gas industry badly damaged, they are forced to seek imports from abroad. The US is opposed to this, and also opposed to the idea of Iran selling oil, though whether they can stop anyone is another matter.