WASHINGTON — As nuclear negotiations with Iran approach another deadline, the United States' main European partners are expressing concern that recent actions by members of Congress have undermined Western credibility in the talks and given Iran more leverage.

Responding to a question from Al-Monitor during a visit to Washington on March 12, German Foreign Minister Frank-Walter Steinmeier said the letter signed by 47 Republican senators warning Iran that a deal would not outlive the Obama administration unless approved by Congress has made an already difficult process “a little more difficult now.”

In the past decade of off-and-on negotiations with Iran, Steinmeier said, the “advantage we [the West] had was to be found in the fact that we said we were credible” and the onus had been on Tehran to prove its ability to live up to its promises. Now, “all of a sudden, Iran is in a position to turn to us and ask 'Are you credible?'” Steinmeier said at the Center for Strategic and International Studies. “We’re not talking about minor details of minor importance.”

Senior French and British diplomats echoed Steinmeier’s concerns and urged Congress to stop interfering in negotiations that may be on the verge of yielding a political framework for a comprehensive deal.