WASHINGTON — The Obama administration, seeking to cut off the flow of oil to the government of President Bashar al-Assad, on Wednesday imposed penalties on five people and six companies it said were defying American sanctions and helping Syria’s government attack its own citizens.

Treasury Department officials said the companies were providing specialty fuels and oil for use by the government to carry out its military campaign. The financial penalties come as President Obama continues to face questions about whether his military campaign against Islamic State militants in Syria is indirectly helping Mr. Assad in the country’s raging civil war.

The officials described an elaborate scheme by which companies based in Syria, the United Arab Emirates, Switzerland and the Netherlands sought to flout the sanctions by falsifying records and mislabeling cargo containing specialty petroleum products bound for Syria.

“So long as the illegitimate Assad regime carries out brutal attacks on its own people, we will seek to disrupt its critical military support networks and increase the financial and economic pressure it faces,” David S. Cohen, the Treasury under secretary for terrorism and financial intelligence, said in a statement.