US sanctions prohibit material support to Syria. Namely, all assets subject to US jurisdiction of the individuals and entities added to the Specially Designated Nationals List (SDN List), and of any other entities blocked by operation of law due to sanctions, are frozen, while US persons are prohibited from dealings with them.
According to the US, the designations target critical actors, including a Syrian national and his Russia-based company. The US said that the man and his company delivered oil from Iran to Syria.
Treasury Secretary Steven Mnuchin stated:
We are issuing an Advisory today identifying the grave risks to the maritime community if they participate in shipping oil to the Government of Syria.
Commenting on the sanctions, Financial Intelligence, Sigal Mandelker, noted that the US will impose sanctions against any party involved in shipping oil to Syria, or seeking to avoid US sanctions on Iranian oil.
On 8 May, U.S. announced the decision to withdraw from participation in the JCPOA (Joint Comprehensive Plan of Action) and to re-impose US nuclear-related sanctions. The announcement triggered global discussions about implications on international trade, especially in shipping. Following this decision, in August, President Trump issued an Executive Order which essentially re-imposed the secondary sanctions against Iran.