Senior US lawmakers expressed sharp frustration with the Obama administration's call to delay new sanctions against Iran on Wednesday, underscoring the difficult sales job the Democratic president has as he pursues a rapprochement with Tehran.Vice President Joe Biden, Secretary of State John Kerry and other top officials visited Capitol Hill to warn senators that implementing the new measures could scuttle delicate talks between Iran and world powers over Tehran's nuclear program."The risk is that if Congress were to unilaterally move to raise sanctions it could break faith in those negotiations and actually stop them and break them apart," Kerry told reporters before the closed-door briefing. But some key lawmakers said after the meeting that they had not been convinced."It was a very unsatisfying briefing," said Senator Bob Corker, the top Republican on the Senate Foreign Relations Committee.However, Corker, also a member of the Senate Banking Committee, which oversees the sanctions measures, said he had not yet made up his mind about whether they should go ahead now.Democratic Senator Tim Johnson, the banking committee's chairman, said he was still undecided about whether to go ahead.Senator Robert Menendez, a Democrat who is chairman of the foreign relations committee and a member of the banking committee, still wants the new sanctions, a spokesman said after the briefing. President Barack Obama's administration wants a "temporary pause" on new sanctions on Iran to allow diplomats from the United States and five other world powers to negotiate with Tehran and test whether it might be possible to resolve a decade-long standoff over its nuclear program."We have the unity of the P5+1, Germany, Great Britain, France ... and Russia, China and the United States are all agreed on this proposal that's on the table," Kerry said."If all of a sudden sanctions were to be increased, there are members of that coalition who have put it in place who would think that we are dealing in bad faith, and they would bolt. And then the sanctions would fall apart," he said.But Obama's diplomacy with Iran has been greeted with skepticism from many quarters, including US allies Israel and Saudi Arabia, as well as among Democrats and Republicans on Capitol Hill, adding a new element to the White House's diplomatic calculations.Negotiators failed to reach an agreement during weekend talks in Geneva. A new round of talks starts on November 20.Western nations fear that Iran's nuclear program is aimed at developing nuclear weapons, while Tehran says it is purely peaceful. But Iran's refusal to halt sensitive nuclear work has drawn tough sanctions targeting the oil exports that are its lifeblood.CONGRESS MORE HAWKISH ON IRAN THAN WHITE HOUSE The package of even tighter sanctions has been making its way through Congress, where lawmakers, including many of Obama's fellow Democrats, generally take a harder line on Iran than the administration.The Republican-controlled House of Representatives passed its bill on July 31, just days before Iran's new president, Hassan Rouhani, took office. Rouhani was elected in June on a platform of conciliation, saying he wanted to ease Iran's international isolation.Senators have been debating behind closed doors their version of the bill, which could slash Iran's oil exports to no more than 500,000 barrels per day and reduce the ability of the Obama administration to waive sanctions.Although Congress has no power to stop the negotiation process, passage of the new sanctions could make things more difficult for Iranian negotiators trying to make the case for concessions to the West, said Aaron David Miller, a former Middle East peace negotiator who is now a vice president at the Woodrow Wilson Center in Washington.More sanctions from Congress could be taken as a sign that Americans want confrontation, not cooperation with the Islamic Republic."It would send an unmistakable signal that he's (Obama's) going to have a hard time selling this agreement," Miller said. "It's going to create huge domestic constraints for those in Iran ... who presumably want to reach a negotiated settlement and want to see sanctions lifted."International oil companies also worry that stricter US sanctions could harm their business with Iran, a factor that could damage consensus in the talks.However the banking committee acts, some senators said they might sidestep the panel and insert a tough new Iran sanctions measure into the annual defense authorization bill, which Obama would find hard to veto.Republican Senator Lindsey Graham, a member of the Senate Armed Services Committee, said that was one possible route for new sanctions. He dismissed Kerry's assertion that new sanctions would torpedo the Geneva talks."I don't buy that one bit quite frankly. What got us here is the sanctions," he said. "We should stop their enrichment, we should freeze and begin to dismantle these centrifuges before you inject cash" into the Iranian economy, he said.But it was not immediately clear that any such effort could win enough support in the Democratic-controlled Senate to move ahead.