WASHINGTON — The Treasury Department on Wednesday announced sanctions against a Syrian construction executive it said was helping President Bashar al-Assad buy oil from the Islamic State, as well as against a Russian businessman who leads the World Chess Federation and who is accused of having financial dealings with the Syrian leader.

The moves are the latest attempt by the administration to financially squeeze Mr. Assad, who President Obama has said must step down to bring an end to the four-year-old Syrian civil war, and to choke off the Islamic State’s cash flow. It is also the first time that the American government has issued sanctions against an individual for helping Mr. Assad do business with the militant group.

The actions emphasize an argument that the administration has increasingly been making about Mr. Assad as it seeks to press Russia to abandon its backing for him: that although the Syrian president professes to be at war with Islamist terrorists, he has a symbiotic relationship with the Islamic State that has allowed it to thrive while he has clung to power.

“The Syrian government is responsible for widespread brutality and violence against its own people,” said Adam J. Szubin, the top Treasury official for terrorism and financial intelligence. “The United States will continue targeting the finances of all those enabling Assad to continue inflicting violence on the Syrian people.”