Psaki said during a Monday briefing with reporters imposing new sanctions on Iran would unravel the unity among multilateral negotiating partners.
"We do feel that putting new sanctions in place during the course of negotiations, even those that are delayed, would be counterproductive," she said.
U.S. Sen. Lindsey Graham, R-S.C., a member of the Senate Armed Services Committee, told CNN a bipartisan group of lawmakers may introduce legislation that would impose more sanctions on Iran.
"We'll do sanctions tied to the end game where the relief will only come if they stop the enrichment program, dismantle [a nuclear] reactor and turn over the enriched uranium," he said Monday.
November negotiations between Iran and members of a multilateral negotiating team resulted in an interim deal where Iran agrees to certain concessions on the enrichment of nuclear material in exchange for relief from some Western-imposed economic sanctions.
Iran and its negotiating partners will spend the next six months finding ways to address comprehensive concerns about Iran's nuclear ambitions.
Iranian Foreign Ministry spokeswoman Marziyeh Afkham was quoted by the semiofficial Fars News Agency as saying the interim deal may collapse if new sanctions are imposed.