Senate and House lawmakers from both parties are set to urge President Joe Biden to step up efforts to deter Iran from pursuing a nuclear weapon, including working with European allies to prepare to initiate the snapback of United Nations sanctions on Iran.

Two as-yet-unreleased congressional letters, led by Sens. Kirsten Gillibrand (D-NY) and James Lankford (R-OK) and Reps. Abigail Spanberger (D-VA) and Dan Crenshaw (R-TX), are a lobbying priority for AIPAC’s Policy Summit in Washington. The letters, which vary somewhat in their precise content but carry a consistent message, were obtained by Jewish Insider.

“It is imperative today that we strengthen our efforts to deter Iran from achieving nuclear weapons capability. We must make Iran understand, in no uncertain terms, that further advances in its nuclear program will be met with unified international action,” the Senate letter reads. “We urge you to restore this posture of deterrence and provide leadership to strengthen the resolve of the international community.”

Both letters argue that the administration should work with allies to implement the snapback of U.N. sanctions if Iran crosses the 90% purity threshold needed for weapons-grade enrichment. Under the Trump administration, the U.S. attempted to initiate the snapback device in the 2015 nuclear deal allowing any participant to unilaterally reimpose U.N. sanctions on Iran, but the effort was rejected by other signatories following the U.S. withdrawal from the deal. Action by the European signatories to the deal — the United Kingdom, Germany and France — would be necessary to implement snapback sanctions, but they have been reluctant to do so.

Biden administration officials have said that diplomacy remains their preferred path to putting Iran’s nuclear program in check, although negotiations have been publicly stalled for months. The Senate letter casts a skeptical tone on such efforts.

“Congress stands united behind the long held bipartisan position that Iran must never be allowed to obtain a nuclear weapon,” the Senate letter reads. “It is crucial for your administration to remain aligned with Congressional efforts related to Iran’s nuclear program and not agree to a pact that fails to achieve our nation’s critical interests. We urge you to take meaningful steps to curb Iran’s destabilizing activities and deter the regime from pursuing this nefarious ambition any further.”

The House letter sends a similar message, calling for a stronger deterrent message.

“The United States must increase its efforts to prevent Iran from obtaining nuclear weapons capability and to communicate to Iran, without any ambiguity, that any further progress in its nuclear program will result in severe consequences,” it reads. “Iran cannot be allowed to advance its nuclear program with impunity.”

Pursuing snapback sanctions, the letter continues, “would send a powerful message of our unified commitment to deterring Iran’s nuclear program. We also believe it would convey the message to Iran that they will not be able to reap the benefits of international sanctions relief if they decide to operate outside of their nuclear commitments.”

Both letters further highlight concerns about Iran’s growing ties to Russia and China, linking the Iranian nuclear issue to other major global challenges for the U.S. The lawmakers also note that Iran was recently found to have enriched uranium to nearly 84% purity, and cite a recent public statement by Under Secretary of Defense for Policy Colin Kahl that Iran could enrich enough uranium for a nuclear weapon in 12 days.