By Arshad Mohammed
WASHINGTON (Reuters) - Iran will be widely seen to be responsible if a comprehensive deal to curb its nuclear program in exchange for sanctions relief is not reached, the top U.S. negotiator said on Thursday.
U.S. Under-Secretary of State Wendy Sherman also said major powers negotiating with Iran have offered it ideas that are "equitable, enforceable and consistent with Tehran's expressed desire for a viable civilian nuclear program."
Britain, China, France, Germany, Russia and the United States are seeking to reach a deal with Iran by Nov. 24. Sherman said Iran's best chance to escape economic sanctions was to strike an agreement before that deadline.
In a speech, Sherman said the United States and the other major powers were prepared to reach an agreement and suggested it would ultimately be seen to be Iran's fault if one did not materialize.
"We hope the leaders in Tehran will agree to the steps necessary to assure the world that this program will be exclusively peaceful and thereby end Iran's economic and diplomatic isolation and improve further the lives of their people," she said.
"If that does not happen, the responsibility will be seen by all to rest with Iran," Sherman added.
Iran's best chance to have sanctions relief is to strike a deal with major powers in the next month that ensures its nuclear program cannot yield a bomb, she said.
"Our goal now is to develop a durable and comprehensive arrangement that will effectively block all of Iran's potential paths to a nuclear weapon," she told a conference organized by the Center for Strategic and International Studies and Syracuse University's Maxwell School of Citizenship and Public Affairs.
Such an arrangement, she added, would prevent Iran from producing fuel for a bomb with uranium or plutonium and would have inspections and monitoring that offered the best chance to prevent Iran from covertly processing these materials.
"If Iran truly wants to resolve its differences with the international community -- and facilitate the lifting of economic sanctions -- it will have no better chance than between now and November 24," she added.
"This is the time to finish the job."
It is unclear whether the deadline, which has already been extended once, from July 20, will be met.
Sherman suggested there may have been an inordinate focus on the number and quality of centrifuges that Iran might be allowed to spin under any comprehensive deal, saying the negotiation "is a puzzle with many interlocking pieces."
She argued that "the status quo" on Iran's uranium enrichment capacity was not acceptable because of the "thick cloud of doubt" cast by what she described as Tehran's past violations of the Nuclear Non proliferation Treaty, secret nuclear-weapons related activities and lack of transparency.
"The world will decide to suspend and then lift nuclear-related sanctions only if and when Iran takes convincing and verifiable steps to show that its nuclear program is and will remain entirely peaceful," she said. "That is a reasonable standard that Iran can readily meet."
(Additional reporting by Lesley Wroughton; Editing by Diane Craft)