VIENNA--The U.S. and other world powers said it will be virtually impossible to reach a comprehensive deal to curb Iran's nuclear activities by the Monday night deadline, and they would favor extending the talks.

U.S. officials are arguing that preserving improved relations with Tehran is preferable to a breakdown in more than a year of direct negotiations and a potential escalation of tensions in an already-fractious Middle East.

However, an extension looks headed for stiff resistance from both Democratic and Republican U.S. lawmakers, who are questioning what the White House believes it can achieve in a few additional months of talks that they couldn't in a year.

U.S. Secretary of State John Kerry backed the idea of an extension even without a core political document that would lay out key terms of the accord--something that could have indicated a final agreement was near.

"Our focus remains on taking steps forward toward an agreement. But it is only natural that just over 24 hours from the deadline, we are discussing a range of options...An extension is one of those options," said a senior U.S. official.

"It should come as no surprise that we are also engaged in a discussion of the options with the Iranians. This does not mean that we are not continuing to discuss the broad range of difficult issues and working to make progress on all the issues that need to be part of a comprehensive agreement," the official added.

The aim of the comprehensive agreement is to constrain Tehran's nuclear program in exchange for an easing of Western sanctions.

American officials have said in recent weeks that they believe Tehran could quickly ramp up its production of nuclear fuel, and potentially sabotage U.S. efforts to combat Islamic State militants in Iraq and Syria, if the negotiating process ends.

The U.S. and its negotiating partners entered into an interim agreement with Iran last year that suspended parts of its nuclear program in return for an easing of some sanctions.

Independent experts tracking Iran's program, however, voiced skepticism Sunday that additional talks would allow Iran and the West to address their differences.

"I don't think it's going to get any easier to overcome the obstacles if they couldn't be overcome in the last four months of pretty intense negotiations," said Mark Kirkpatrick, a former State Department official who closely tracks Iran's nuclear program for London's International Institute for Strategic Studies.

He added, however: "Certainly an extension is far, far better than a return to square one of more centrifuges and more sanctions. As long as Iran's enrichment program remains capped it's to the advantage of the U.S. and its allies."

Some senior Western diplomats involved in the Vienna talks also doubted Tehran would be willing to significantly change its position in the coming months.

"The [Iranian] negotiating team that is in Vienna--they are open, nice people, who are doing the best they can," said the official. "The problem is that you have some who I'm not sure have the mandate to get to the place they need."

Mr. Kerry held four hours of negotiations on Sunday with his Iranian counterpart, Javad Zarif, in a bid to push the diplomatic process forward, according to U.S. officials. The foreign ministers of the U.K., France, Russia, Germany, and Saudi Arabia also visited Vienna, and China's top diplomat, Wang Yi, is expected to arrive Monday morning.

Iran is formally negotiating with the five permanent members of the United Nations Security Council plus Germany, a diplomatic bloc called the P5+1.

The major stumbling blocks to an agreement remain the future capacity of Iran to produce nuclear fuel and the pace at which Western sanctions will be removed, according U.S., European and Iranian officials.

Tehran has been seeking to produce nuclear fuel at an industrial scale, using tens of thousands of fast-spinning centrifuge machines to enrich uranium. The U.S. and its European partners have been seeking to limit Tehran's capacity to a few thousand of the machines.

Iran also wants a rapid waiving of U.S., European Union and U.N. sanctions that have crippled its finances over the past five years. The Obama administration is seeking a gradual repeal tied to Iran implementing the commitments it makes to cap its nuclear work.

Also in question is the future of Iran's heavy water reactor in the city of Arak, which is capable of producing plutonium usable in weapons. The U.S. and U.N. are also pressing Tehran to cooperate with U.N. inspectors investigating evidence Iran has conducted secret studies on the development of atomic weapons, something the country has repeatedly denied.

U.S., European and Iranian officials said they have made progress over the past year in negotiations that have crisscrossed the world from Oman to New York. But Obama administration officials increasingly are questioning whether Iran's most powerful political player, Supreme Leader Ayatollah Ali Khamenei, has empowered Mr. Zarif and other negotiators to make the necessary concessions for a deal. Mr. Zarif has repeatedly said he's empowered to negotiate but that Mr. Khamenei is the ultimate decision maker.

The diplomacy over the past year has coincided with the rise of Islamic State militants who have captured large swaths of Iraq and Syria since June.

U.S. officials have repeatedly said the Vienna negotiations are being conducted independently of other regional security issues. But they have also stated that Washington and Tehran have shared interests in combating Islamic State and that tensions between the two countries have been lowered significantly as a result of the direct diplomacy.

Last month, Mr. Obama secretly wrote Mr. Khamenei and suggested the U.S. and Iran could cooperate against Islamic State if the nuclear issue is resolved, according to people briefed on the letter.

Another senior U.S. official said late Sunday that if an extension is needed the Obama administration would make the case that talks have continued to make incremental progress and that given time, the diplomacy could still bring a result.

With Iran's economic situation unlikely to improve, the diplomat said extending the talks would allow the two sides to keep chipping away at the differences and come to an agreement.

"The current process cannot continue forever," the official said.

But "if we've made progress and we are closer than we have ever been, think about the consequences if you walk away?"

Mr. Kerry held a dinner late Sunday with the foreign ministers of the U.K., France and Germany, according to U.S. and European officials, where the parameters of a possible extension were being discussed.

The terms of an extension would be complicated, said these officials, and would likely involve additional economic inducements for Iran. Last year, the P5+1 suspended sanctions on the airline and automotive sectors and released nearly $4.2 billion in Iranian oil assets that were frozen in overseas accounts.

Republican lawmakers, who gained control of the Senate in elections this month, have already stated they would oppose such incentives and they would move to impose new sanctions on Tehran.

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