Biswal defended the waiver decision, saying Iran used to be India's second-largest supplier, but is now fifth or sixth.
India is reliant on imported crude and is reluctant to completely stop imports from Iran. The Rhodium Group, a U.S. consultancy, says India's imports from Iran have reduced from 486,000 barrels per day in January 2012, to 141,000 barrels per day in June 2013.
American sanctions are designed to pressure Iran to curb its nuclear program, which Washington suspects is aimed at producing weapons. Iran has repeatedly insisted it is only for generating electricity and medical research.
The U.S. is pressing countries around the world to cut commercial ties with Iran or face a series of restrictions on what type of business they can conduct in the United States, the world's largest market. But the Obama administration has granted exemptions to a number of mostly Asian countries, also including China Iran's top trading partner and core U.S. allies, Japan and South Korea.
"India has scrupulously adhered to the multilateral sanctions against Iran as mandated by the United Nations," said M. Sridharan, spokesman for the Indian Embassy in Washington. "As we diversify our energy resources and as our oil importers make their own commercial judgment, crude imports from Iran have a steadily declining share in India's total oil imports."
Democratic Sen. Tim Kaine said that imports of U.S. liquefied natural gas could help India reduce its reliance on oil from the Middle East. He urged India to eliminate or dramatically cut purchases from Iran until it makes plain it doesn't have a path to nuclear weaponry but offer to buy more once Iran's provides that assurance.
Biswal, currently assistant administrator for Asia at the U.S. Agency for International Development, is expected to be confirmed as assistant secretary of state for a region straddling Central Asia, Bangladesh, Bhutan, Nepal and Sri Lanka. She said advancing the U.S.-India strategic partnership was of "paramount importance."
She said she was hopeful of progress in fulfilling a landmark 2008 India-U.S. civil nuclear agreement to help satisfy its growing energy needs. But she cautioned it was going to be a "long and tough road" to resolve differences over Indian nuclear liability legislation that effectively has blocked U.S. suppliers from capitalizing on the agreement.
Biswal said India is still grappling with the legacy of the Bhopal disaster when a leak at a pesticide plant run by U.S. company Union Carbide Corp. killed an estimated 15,000 people and affected at least 500,000 more. She said that defined how Indians view nuclear power.
Many survivors of that 1984 disaster are still campaigning for compensation.