London's Guardian newspaper reported last week that Royal Dutch Shell increased the amount of oil it purchased from Iran by 27 percent from May to August compared with the previous three months.
The report added, however, that Shell's activities in Iran didn't explicitly violate sanctions imposed by the United States and European Union on Iran's energy sector.
The U.S. State Department said last week that BP and Shell are no longer selling jet fuel to Iran Air, while Shell, Total, Spanish energy company Repsol and Italy's Eni all abandoned work in the upstream sector.
UANI applauded the energy companies, including Shell, for complying with the sanctions and ending their investments in Iran but called for a complete halt to all activity in the energy-rich Islamic republic.
"These European firms must recognize that their decisions to conduct business in Iran affect Iran's ability to develop nuclear weapons," UANI President Mark Wallace said in a statement. "In particular, Royal Dutch Shell must come clean and make transparent the full extent of and risks associated with its business in Iran."
Shell announced in May that it stopped selling gasoline to Iran as the threat of sanctions mounted.