Iran says U.S. economic sanctions against the country have made it harder for the national Iranian airline to refuel its planes abroad, a situation Tehran calls “inhumane.”

Several Western companies have stopped selling fuel to Iran Air planes at European airports in recent months to comply with U.S. sanctions prohibiting the export of refined gasoline to Iran.

Iranian Foreign Ministry spokesman Ramin Mehmanparast said Tuesday the fuel sanctions against Iran Air violate international norms.

The United States toughened its Iran sanctions last week, imposing new penalties on Iran Air on suspicion of transporting military-related equipment on behalf of the Iranian government.

The United States, European Union and their allies have imposed a series of unilateral sanctions on Iran to pressure it into stopping sensitive nuclear work that they fear could be used to make atomic bombs.

Iran also is subject to four rounds of U.N. sanctions for refusing to suspend its program of uranium enrichment, which Tehran says is meant only for peaceful use.