"You'll hear over the next six months a lot more about our efforts on sanctions" against Iran, Ambassador William Kennard told a conference organised by the European Policy Centre think-tank in Brussels.
US President Barack Obama promised after coming to power a year ago that he was ready for "constructive ties" with Tehran, speaking of "a new era of engagement."
However, Kennard said Wednesday, "we also realize that having put some serious offers on the table" in order to have "a pathway to a diplomatic solution, those offers can't be available for ever."
"We can't afford to let the situation fester," he stressed.
Iran has ignored repeated UN Security Council ultimatums to suspend uranium enrichment, the sensitive process which makes nuclear fuel but in highly extended form can also produce the fissile core of an atomic bomb.
It is already under three sets of UN sanctions. Major powers, including the United States, are to meet in New York on Saturday to discuss proposals for a fourth.
Tehran maintains that its nuclear programmes is for strictly civil uses.
