Shortly after President Barack Obama met with top senators to talk about Iran sanctions Tuesday, several of those lawmakers joined a letter expressing concern about what an nuclear arms agreement with that country may require.

The letter was signed by six senators including three participants in Tuesday’s White House talks: Democrats Charles E. Schumer of New York and Robert Menendez of New Jersey, as well as Arizona Republican John McCain.

“While the interim agreement may suggest that Iran could be willing temporarily to slow its pursuit of a nuclear weapons capability, it could also allow Iran to continue making some progress toward that end under the cover of negotiations. This does not give us confidence that Iran is prepared to abandon unambiguously its nuclear weapons pursuit altogether, as it must,” the senators wrote.

Menendez is chairman of the Foreign Relations Committee. The three other signatories are Sens. Lindsey Graham, R-S.C., Bob Casey, D-Pa., and Susan Collins, R-Maine.

“Easing sanctions now without real, tangible actions by Iran to roll back its nuclear program would not only diminish this threat of future pressure, it could make it more difficult to maintain the current sanctions regime at a time when many international actors are already eager to lessen their implementation of sanctions,” the senators added. “We feel strongly that any easing of sanctions along the lines that the P5+1 is reportedly considering should require Iran to roll back its nuclear program more significantly than now envisioned.”

The White House said in a statement issued after the roughly two-hour meeting, which included Secretary of State John Kerry and National Security Adviser Susan Rice, that any relief from current sanctions as part of the agreement with the six countries negotiating with Iran over the nuclear program would be reversible and contingent on future progress.

“The President expressed his appreciation for the bipartisan Congressional support for the most effective sanctions regime in history. He reiterated that the purpose of sanctions was and remains to change Iran’s calculus regarding its nuclear program. He indicated that new sanctions should not be enacted during the current negotiations, but that they would be most effective as a robust response should negotiations fail,” the White House said. “The President is determined to prevent Iran from obtaining a nuclear weapon, and firmly believes that it would be preferable to do so peacefully.”

Senate Foreign Relations ranking member Bob Corker, R-Tenn., told reporters at the White House after the meeting that he did not envision any Senate action on ratcheting up Iran sanctions before the Thanksgiving recess, noting the administration position and the decision by Senate Majority Leader Harry Reid, D-Nev., to preclude the free flow of amendments on the defense authorization bill currently on the floor.