US President Barack Obama said Thursday that an interim deal with Iran on its nuclear program would provide only "very modest relief" from the sanctions that have crippled Tehran's economy.

Obama said in an interview with NBC News that an agreement being fleshed out in high stakes talks in Geneva would keep the bulk of sanctions on the Islamic Republic in place.

"We don't have to trust them. What we have to do is to make sure that there is a good deal in place from the perspective of us verifying what they're doing," Obama said.

"There is the possibility of a phased agreement in which the first phase would be us, you know, halting any advances on their nuclear program, rolling some potential back, and putting in place ... some very modest relief, but keeping the sanctions architecture in place."

The president said that if necessary such limited relief from sanctions could be reversed if it was judged Iran was not living up to its end of the bargain.

"If they're not willing to go forward and finish the job of giving us assurances that they're not developing a nuclear weapon, we can crank that dial back up," Obama said.

Reports from Geneva, where Iranian negotiators are in talks with key world powers including the United States, suggest an interim deal will include limits on Iran's enrichment capacity, verification steps and restrictions on its stockpiles of atomic material and its nuclear facilities.

Such an agreement would be designed to build confidence on both sides with a view to concluding a final deal that would verifiably convince the West that Iran is not building nuclear weapons.