TOUGH CHALLENGES AHEAD FOR US

Thursday's sanctions against Iran were not the first time this year. The Treasury on March 23 imposed sanctions on an Iranian entity and Iranian individuals for "malicious cyber-enabled activities."

On Jan 12, it also imposed sanctions on 14 individuals and entities over Iran's alleged human rights abuses and ballistic missile program.

A day after Trump's exit decision, White House spokesperson Sarah Sanders said "we are preparing to add additional sanctions that may come as early as next week."

Peter Harrell, an adjunct senior fellow at the Center for a New American Security, said that given international opposition to the US withdrawal and "scant international support for renewed sanctions," Trump would face formidable challenges.

"Perhaps the biggest diplomatic challenge Trump would face in reinstituting sanctions on Iran is convincing buyers of Iranian oil to reduce their purchases," said Harrell, also a lawyer who advises on sanctions compliance.