VIENNA - A senior Iranian official indicated on Tuesday that progress was being achieved in expert-level talks between Tehran and six world powers over the implementation of a landmark nuclear deal, as US Secretary of State John Kerry urged Congress to hold off on imposing new sanctions on the Islamic Republic.Despite noted progress, Iranian Ambassador Reza Najafi said that the meeting on the deal's implementation, which began on Monday at the Vienna headquarters of the International Atomic Energy Agency (IAEA), the UN nuclear watchdog, would continue for a third day on Wednesday.The goal is to work out nitty gritty details of implementing the November 24 interim accord under which Iran will curb its disputed nuclear program in return for some easing of sanctions that have battered its oil-dependent economy. Asked whether good progress was being made, Najafi told reporters: "Yes. We are going to continue tomorrow." Najafi is Iran's ambassador to the IAEA, which will have a key role in verifying that Iran fulfills its side of the agreement.Officials from Iran, the United States, China, Britain, France, Germany, Russia, the European Union and the IAEA attended the meeting in the Austrian capital.The preliminary accord is seen as a first step towards resolving a decade-old standoff over suspicions Iran might be covertly pursuing a nuclear weapons "breakout" capability, a perception that has raised the risk of a wider Middle East war. Western diplomats said detailed matters not addressed at the November 20-24 talks in Geneva must be ironed out before the deal can be put into practice.These include how and when the IAEA, which regularly visits Iranian nuclear sites to try to ensure there are no diversions of atomic material, will carry out its expanded role. A start to sanctions relief would hinge on verification that Iran was fulfilling its side of the accord, they said.The deal was designed to halt Iran's nuclear advances for a period of six months to buy time for negotiations on a final settlement of the standoff. Diplomats say implementation may start in January after the technical details have been settled.Scope for easing the dispute peacefully opened after the June election of a comparative moderate, Hassan Rouhani, as Iranian president. He won in a landslide by pledging to ease Tehran's international isolation and win relief from sanctions that have severely damaged the oil producer's economy. Meanwhile, US Secretary of State Kerry appealed to skeptical US lawmakers on Tuesday not to impose new sanctions on Tehran.Republicans and Democrats in the House of Representatives raised questions about the interim deal.The Obama administration hopes that the interim agreement, slated to last six months but renewable for a further six months by mutual consent, will provide time to negotiate a final deal with the Iranians within a year."We are asking you to give our negotiators and our experts the time and the space to do their jobs and that includes asking you while we negotiate that you hold off imposing new sanctions," Kerry told the House Foreign Affairs Committee. "I am not saying never ... If this doesn't work, we are coming back and asking you for more. I am just saying not right now," he added. "This is a very delicate diplomatic moment."On Monday, Iranian Foreign Minister Javad Zarif was quoted as saying new US sanctions - which the United States promised not to impose in the November 24 agreement - would kill the deal."We are at a crossroads, we are at one of those hinge-points in history - one path could lead to an enduring resolution in international communities' concerns about Iran's nuclear program, the other path could lead to continued hostility and potentially conflict," Kerry said.The Republican House passed a new package of sanctions by a vote of 400 to 20 at the end of July. That bill seeks to cut Iran's oil exports to near zero over the course of a year to try to reduce the flow of funds to the nuclear program.However, the Democratic-led US Senate has moved much more slowly, in part at the administration's request, and it remains unclear whether Congress may be willing to put the November 24 deal at risk by passing fresh sanctions.In a sign of bipartisan skepticism over the November 24 agreement, Eliot Engel, the top Democrat on the House panel, told Kerry he thought the administration would have more leverage with more sanctions."I think it could potentially strengthen your hand with a good cop, bad cop scenario," Engel said.Kerry suggested new sanctions would be seen as an act of bad faith because the United States agreed under the November 24 deal not to impose fresh sanctions. He also said they could undermine the unity of the six major powers who negotiated that deals.