European allies are blasting a letter from GOP senators to Iran that threatened to void any nuclear deal, saying it is creating "mistrust" in the delicate negotiations.

“This is not just an issue of American domestic politics, but it affects the negotiations we are having in Geneva,” German Foreign Minister Frank-Walter Steinmeier said Thursday in Washington before a meeting with lawmakers, according to Agence France-Presse.

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“Obviously, mistrust is growing on ... the Iranian side if we are really serious with the negotiations,” he added.

Steinmeier said he hopes “the letter no longer causes any disturbance in the negotiations” next week in Lausanne, Switzerland.

The Associated Press reported that diplomats from other nations involved in the talks have also criticized the letter.

A senior French diplomat said the GOP letter made negotiations an uphill battle for the U.S. And an official with the British foreign office stressed that outside observers should not interfere in the complex bargaining. Both diplomats spoke on the condition of anonymity given the delicacy of the talks.

The letter was led by Sen. Tom CottonTom Bryant CottonBarrett fight puts focus on abortion in 2020 election COVID outbreak threatens GOP's Supreme Court plans This week: Coronavirus complicates Senate's Supreme Court fight MORE (R-Ark.). In it, lawmakers warned Iran that Congress would weigh in on any nuclear deal and that lawmakers and future presidents could void the terms.

But the effort sparked a fierce backlash from the White House and Democrats, putting Republicans on the defensive. The White House on Tuesday said the letter to a foreign government in the midst of negotiations was “without precedent.”

Ayatollah Ali Khamenei, Iran’s supreme leader, also attacked the letter on Tuesday as an example of American “backstabbing.”

The Obama administration hopes to reach an outline for a deal with Iran by March 24. A final agreement is due on June 30.

The U.S. part of the P5+1 group — with Britain, China, France, Germany and Russia — that is negotiating with Iran. The sides are seeking a deal that would end crippling economic sanctions on Iran in exchange for Tehran abandoning its pursuit of nuclear weapons.