WASHINGTON - European allies are joining the Obama administration in criticizing Republican congressional interjection into nuclear negotiations with Iran, saying that an open letter from Republican senators to Iranian leaders has been counterproductive and has come at a particularly sensitive time in the talks.

"Suddenly, Iran can say to us: 'Are your proposals actually trustworthy, if 47 senators say that no matter what the government agrees to, we can subsequently take it off the table?' " German Foreign Minister Frank-Walter Steinmeier said Thursday during a visit to Washington.

"This is no small matter we're talking about," Steinmeier warned in remarks at the Center for Strategic and International Studies. "This is not just an issue of American domestic politics." Germany, France, and Britain, along with Russia and China, are U.S. negotiating partners in the Iran talks.

Iran's supreme leader, Ayatollah Ali Khamenei, suggested Thursday that political divisions in the United States made Iranian negotiators question the Obama administration's ability to follow through with any agreement.

"Of course I am worried, because the other side is known for opacity, deceit, and backstabbing," Khamenei said, according to Iran's Mehr news agency.

European allies are alarmed by U.S. political tensions on a host of foreign policy issues. President Obama has so far resisted demands from a bipartisan congressional majority to send lethal military equipment to Ukraine. Germany and France, which helped negotiate a sputtering cease-fire between the Ukrainian government and Russian-backed separatists, have said Western arms shipments would only escalate the conflict and undermine a possible solution.

On Thursday, Republicans struck back at European criticism. Sen. Ted Cruz (R., Texas) said that what he considered U.S. and European capitulation to Iran was reminiscent of Western appeasement of Adolf Hitler.

"I believe we are at a moment like Munich in 1938," Cruz said on the Hugh Hewitt radio show.

Sen. John McCain (R., Ariz.) extended the World War II metaphor to Ukraine in a direct attack on Steinmeier. "The foreign minister of Germany is the same guy that refuses, in his government, to enact any restrictions on the behavior of [Russian President] Vladimir Putin, who is slaughtering Ukrainians as we speak. He has no credibility."

Steinmeier, McCain said, "is in the Neville Chamberlain school of diplomacy," a reference to the British prewar prime minister, who signed the 1938 Munich Agreement conceding the Sudetenland region in Czechoslovakia to Hitler's Germany.

Secretary of State John Kerry told lawmakers Wednesday that the letter to Iran by 47 Republican senators "risks undermining the confidence that foreign governments in thousands of important agreements commit to. It purports to tell the world that if you want to have any confidence in your dealings with America, they have to negotiate with 535 members of Congress."

The letter warned Iran that any nuclear agreement signed by Obama could be revoked "by the stroke of a pen" by any future president and that Congress could modify its terms "at any time."

Vice President Biden this week called the letter "a highly misleading signal to friend and foe alike that our commander-in-chief cannot deliver on America's commitments - a message that is as false as it is dangerous."

Kerry and the other negotiators will meet with Iranian officials next week in Lausanne, Switzerland, for another round of talks. The administration has said a framework for a deal must be agreed to by the end of this month if technical details were to be completed before the negotiating deadline at the end of June. Negotiators have said that deadline would not be extended.

U.S. and European officials have cited recent progress in the negotiations, while cautioning that a deal might not be possible. Outstanding issues are said to include the future status of Iran's underground nuclear enrichment facility at Fordow, near the city of Qom, and aspects of Iran's nuclear research and development program.

Obama has said the goal was to block Iran's path to a nuclear weapon in a lasting and verifiable way. Opponents have insisted that Iran's nuclear capability - which Tehran says is designed only for legitimate, peaceful energy purposes - must be destroyed.

Even before Steinmeier's visit, Germany, Britain, and France had repeatedly expressed concern about congressional interference in the talks. Ambassadors from the three Western European countries have been a frequent presence on Capitol Hill, trying to persuade lawmakers to hold off on new Iran sanctions or any other legislation related to the negotiations while they are underway.

Following publication of the letter Sunday night, French Ambassador Gerard Araud, a diplomat of long experience in this country and a prolific user of social media, posted a Twitter link to the letter and his own comment that "for a foreigner, Washington can be full of surprise."

In London on Tuesday, British Foreign Secretary Philip Hammond told a parliamentary committee that new legislation on Iran "could become a spanner in the works" and "have an unpredictable effect on leadership opinion and public opinion in Tehran."