The White House welcomed Sunday's implementation of an EU embargo on Iranian oil, calling it "an essential part" of an international response to Tehran's nuclear programme.
President Barack Obama's Press Secretary Jay Carney in a statement said the United States and the European Union were committed to holding Iran accountable "for failing to meet its international obligations."
"With this decision, our partners in the EU have underscored the seriousness with which the international community views the challenge of Iran's nuclear ambitions," Carney said.
"This action is an essential part of our concerted diplomatic efforts to present Iran with a clear choice between isolation or meeting its obligations."
With expert-level talks set for Tuesday in Istanbul, he said, Iran "must take concrete steps toward a comprehensive resolution of the international community's concerns with Iran's nuclear activities."
Iran reacted with anger as the EU embargo went into effect, saying the measure would hurt talks with world powers over its sensitive nuclear activities.
Iranian leaders have said they will forge ahead with their atomic programme, regardless of Western and UN Security Council restrictions, amid concerns that the Islamic republic is secretly building a homegrown nuclear arsenal.
On Thursday, the United States exempted China and Singapore from sanctions over purchases of oil from Iran, saying that major economies were united in pressuring Tehran.