Treasury Secretary Janet Yellen told lawmakers on Thursday that U.S. sanctions on Iran have not had as much of an effect as hoped in curbing Tehran’s malign activities, and that the U.S. needs to turn to other tools in this effort.

Testifying before the House Appropriations Committee, Yellen told lawmakers that the U.S. has leveled “the toughest possible sanctions on Iran” and undertaken ongoing efforts to strengthen them, creating a “real economic crisis in the country.” But, she continued, those punishing conditions have not fully deterred the regime in Tehran.

“Iran is suffering greatly economically, because of the sanctions. I would say the same thing is true of North Korea,” she said. “On the other hand, if you were to ask, ‘Has that forced a change in behavior?’ The answer is much less than we would, ideally, like.”

“Sometimes a regime is so committed to a program, that even when the population of that country is suffering immensely because of sanctions we’ve imposed, they continue to prioritize activities that are the ones we’re trying to stop,” she explained.

Yellen said the Treasury Department needs to turn to “a broader tool kit” to head off the Iranian regime.

“Sanctions can play a role in changing behavior but they may not be sufficient,” she said. 

The Trump administration turned to a “maximum-pressure” sanctions campaign against Iran in 2018 after pulling out of the 2015 Joint Comprehensive Plan of Action. Under those sanctions, Iran’s nuclear development and malign activities have continued and in some cases increased.

Some conservatives have accused the Biden administration of failing to fully enforce the sanctions in an effort to reenter the nuclear deal; the administration has insisted it continues to enforce the maximum pressure regime and notes that it has leveled additional sanctions.