WASHINGTON – Comprehensive nuclear talks between world powers and Iran have yet to begin, but diplomats from Iran and the United States are already trading harsh words.America’s chief nuclear negotiator has “hindered” the diplomatic process with her testimony before Congress this week, Iranian Foreign Minister Mohammad Javad Zarif charged on Wednesday, calling on the US diplomat to “stick to reality.”Testifying on Tuesday before the Senate Foreign Relations Committee, US Under Secretary for Political Affairs Wendy Sherman said that the Islamic Republic had “no need” for an underground, mountain-fortified nuclear facility enriching uranium beyond any known civilian nuclear purpose.While conditioning her statement by saying she would not negotiate with Iran in public, Sherman said the US would seek to close that facility in Fordow, as well as a heavy-water plutonium facility in Arak.Sherman, who previously negotiated with North Korea over its nuclear program, has led negotiations with Iran since public talks began in September.The exchange marks a notable deterioration in an atmosphere previously described by senior US administration officials as cordial.“Those who know our peaceful objectives are also aware that we will not negotiate about our facilities,” Zarif said. “Ms. Sherman should stick to the reality and stop speaking of impossible things even if it is only for domestic consumption..., since reaching a solution can be hindered by such words.”In recent days, Sherman and Zarif have characterized each others’ statements as efforts to play to domestic audiences.The Iranian foreign minister, as well as President Hassan Rouhani, have stated several times this month that their government will never dismantle its extensive nuclear enrichment network, spanning more than 20,000 centrifuges across many facilities.Recent statements from Iran’s leaders represent their “maximalist position” upon entering comprehensive negotiations, Sherman told the Senate panel, adding that she “wouldn’t expect any less.”Sherman said the US hopes for full dismantlement, but that the Obama administration would tolerate a “small, limited enrichment program” – a position vehemently opposed by Prime Minister Binyamin Netanyahu.Dr. Ali Akbar Salehi, the head of Iran’s Atomic Energy Organization, echoed Zarif on Wednesday in an interview with Iran’s state-run news outlet Press TV.“If you look at the word ‘dismantle’ and you look at it in the dictionary, dismantle means to take apart and try to put it into pieces, equipment,” Salehi said. “Well, you can come and see whether our nuclear sites, nuclear equipment and nuclear facilities are dismantled or not.”Salehi said that low-grade nuclear enrichment would actually increase during the negotiating period.“The nuclear facilities are functioning; our enrichment is proceeding, it’s doing its work, it’s producing the 5-percent enriched uranium and those centrifuges that stopped producing the 20% will be producing 5% enriched uranium,” he added. “In other words, our production of 5% [uranium] will increase.”Meanwhile on Capitol Hill, dozens of Democratic congressmen have signed a letter in support of US President Barack Obama’s opposition to new sanctions legislation during the duration of the interim nuclear deal freezing much of Iran’s nuclear work in exchange for sanctions relief, formally known as the Joint Plan of Action.According to the office of its author, Rep. Lloyd Doggett (D-Texas), the letter has more than 70 signatories.That number marks a significant shift in attitudes since before negotiations with Iran began in the autumn. Just a month before Tehran and Washington engaged in a public rapprochement in September, 400 congressman voted to sanction Iran punishingly with a bill that has since become irrelevant.“We understand that there is no assurance of success and that, if talks break down or Iran reneges on pledges it made in the interim agreement, Congress may be compelled to act as it has in the past by enacting additional sanctions legislation,” the letter reads. “At present, however, we believe that Congress must give diplomacy a chance.“A bill or resolution that risks fracturing our international coalition or, worse yet, undermining our credibility in future negotiations and jeopardizing hard-won progress toward a verifiable final agreement, must be avoided,” the letter continues.