Japan imposed sanctions against Iran on Tuesday in line with a UN resolution and said it plans to announce additional punitive measures later this month.
Robert Einhorn, State Department special adviser for non-proliferation and arms control, called for tough measures from Tokyo, which has long been on relatively good terms with Tehran.
"Japan imports a lot of oil from Iran, but the steps we are asking Japan to take would not interfere in any way with Japan's energy security, its imports of oil from Iran," he told a press conference.
He said he had told Japanese officials in an earlier meeting "to look at the measures already adopted by the European Union".
"These are strong measures. Japanese adoption of these strong measures would not adversely affect the economy of Japan," said the official, who earlier visited South Korea to rally support for tough sanctions.
The UN Security Council in June slapped its fourth set of sanctions on Iran over its refusal to halt its uranium enrichment work, part of a nuclear programme which many nations fear masks a drive for nuclear weapons.
The European Union last week announced additional sanctions.