The growing pressure has stirred debate in the West about when Iran will reach its "economic drop dead date" — when its currency reserves are so low that it can't meet its budget or take care of other key tasks, such as intervening to prop up its currency.
Mark Dubowitz, an energy specialist at the pro-sanctions Foundation for Defense of Democracies, says Tehran's refusal to give ground so far shows that "Iranian nuclear physics is beating out Western economics — they're not yet feeling such severe pressure that they feel they have to compromise."
He said it could take until the end of 2013 to bring Iran's economy to wholesale collapse.
The European oil embargo, which goes into effect Sunday, will deprive Iran of 18% of former oil sales. India, Japan and most other major customers already have begun cutting purchases of Iranian oil, partly because they could face U.S. sanctions if they don't.
The Obama administration has waived sanctions against 18 countries that have substantially shifted their oil supply from Iran to Saudi Arabia or other producers. Analysts say it appears likely the administration will also grant a waiver to China, Iran's biggest customer, which cut purchases of Iranian oil sharply in the first quarter.