"Assad's despotic regime is under an international spotlight for using chemical weapons and committing atrocities against innocent Syrian civilians, and they rely on these types of illicit networks to stay in power," said Sigal Mandelker, a top Treasury Department official.
Meanwhile, the US also announced it had determined the Syrian government used chlorine gas in an attack against rebels on May 19, in what Secretary of State Mike Pompeo said was a violation of the convention prohibiting the use of chemical weapons.
The strike took place in Latakia province, in north-west Syria. At the time, the US said it was investigating the reports.
Pompeo said the government has used chemical weapons each year since 2013, when it joined the international convention.
"The United States will continue to pressure the insidious Assad regime to end the violence directed against Syrian civilians," Pompeo said, but did not announce any action planned against President Bashar al-Assad.
The US Treasury Department said it had targeted Maritime Assistance LLC, a front company for OJSC Sovfracht, which was previously sanctioned for operating in Ukraine.
Treasury accused Sovfracht of facilitating the transfer of jet fuel to Russian military operating in Syria in support of Syrian President Bashar al-Assad.
The department also targeted three individuals linked to Sovfracht, as well as identifying five vessels as "blocked property" of Transpetrochart Co Ltd, a previously-sanctioned Russian company that allegedly supported Sovfracht.
"The US is determined to cut off access to the international financial system from those who conspire to violate our sanctions, including those who enable the brutal war machine in Syria," Mandelker added.
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