VIENNA (AP) — Still facing significant differences between the U.S. and Iran, negotiators gave up on last-minute efforts to get a nuclear deal by the deadline Monday and extended their talks for seven more months.

The move gives both sides breathing space to work out an agreement but may be badly received by skeptics in the United States, since it extends more than a decade of diplomatic efforts to curb Iran's nuclear prowess.

International negotiators are worried that Iran is using its nuclear development program as a cover for developing nuclear weapons and they have imposed economic sanctions on Tehran. Iran denies the charge, saying it is only interested in peaceful nuclear programs like producing power.

After a frenetic six days of diplomacy in Vienna, negotiators agreed Monday to nail down by March 1 what needs to be done by Iran and the six world powers it is negotiating with, and by when. A final agreement is meant to follow four months later.

Comments by key players in the talks suggested that not much was agreed on in Vienna beyond the decision to keep talking. The next negotiating round was set for early December but the venue is still unclear.

U.S.-Iran relations have warmed since Iranian President Hassan Rouhani took office last year and the thaw has extended to the nuclear negotiations, which have his strong backing. On Monday, he urged perseverance despite the setback.

"Our logic has gotten closer, many gaps have been eliminated," Rouhani said in a statement. At the same time, he said the sides were "still some distance" from sealing a deal.

Rouhani has occasionally struggled to sell the idea of negotiating with arch-foe America to hardliners at home and he pledged "ultimate victory" for the Islamic Republic in securing a favorable agreement.

Monday's decision appears to benefit Iran. Its nuclear program is left frozen but intact, without any of the cuts sought by the U.S. And while the negotiations continue, so will monthly dole-outs of $700 million in frozen funds that began under the temporary nuclear deal agreed on late last year that led to the present talks.

British Foreign Secretary Philip Hammond said the sides were giving themselves until March to agree on a text "that sets out in layman's language what we have agreed to do." Experts then will be given another four months to "translate that into precise definitions of what will happen on the ground," he told reporters.

Even the new deadline was not immediately clear, with negotiators saying it was July 1, and Hammond fixing it at June 30.

Among other issues, the two sides are haggling over how many — and what kind — of centrifuges Iran should be allowed to have. The machines can enrich uranium from low reactor-fuel level up to grades used to build the core of a nuclear weapon. Washington wants deeper and more lasting cuts in the program than Tehran is willing to give.

Past talks have often ended on an acrimonious note, with each side blaming the other for lack of a deal. But, mindful of hard discussions ahead, U.S. Secretary of State John Kerry focused on praise, in an apparent attempt to maintain a relatively cordial atmosphere at the negotiating table.

Kerry, who arrived Thursday and met repeatedly with Iranian Foreign Minister Mohamad Javad Zarif, said Zarif "worked diligently and approached these negotiations in good faith."

"We have made real and substantial progress and we have seen new ideas surface," Kerry told reporters. "Today we are closer to a deal that will make the whole world, especially our allies in Israel and the Gulf, safer."

Hammond and other foreign ministers of the six powers also sought to put a good face on what was achieved. Hammond spoke of "significant progress," while German Foreign Minister Frank-Walter Steinmeier said only differences about "technical details" remained.

"All the people involved here feel that there really is a chance to find out a way to each other and we are going to take that chance," Steinmeier said about the decision to extend.

But the length of the extension suggests that both sides felt plenty of time was needed to overcome the disputes on how much Iran needed to restrict nuclear activities that could be used to make weapons.

Obstacles far from the negotiating table could also complicate the process.

Democratic Senator Carl Levin of Michigan, chairman of the U.S. Senate Armed Services Committee, said he backed having the talks go longer because it has "maintained the tough sanctions regime" and kept curbs on the Iranian nuclear program.

Many others will likely be critical, however. Members of the new Republican-controlled U.S. Congress that will be sworn in in January have already threatened to impose additional sanctions on Iran and may well have enough votes to overturn an expected veto of such legislation by President Barack Obama.

New sanctions could very well derail the talks, as Iran has signaled they would be a deal breaker. Kerry said he hoped congressional skeptics would "come to see the wisdom of leaving us the equilibrium for a few months to be able to proceed without sending messages that might be misinterpreted."

In Tehran, hardliners fearful that their country could give away more than it gets under any final deal could increase pressure on Supreme Leader Ayatollah Ali Khamenei to break off talks. The talks extension, however, appears to have the approval of Khamenei, the ultimate arbiter in his country.

Tehran residents hoping for relief from sanctions and a reduction in tensions expressed frustration at Monday's decision.

"The West is making a big mistake," said high school teacher Abbas Hoseini. "Instead of working with Iran and a close engagement, they are pushing Iran toward Russia and China."

An extension was widely expected as the deadline approached with neither side having the appetite for new confrontation that would renew the threat of military action against Iran by Israel, and potentially the U.S., as well as tighten the sanctions regime on Tehran.

Alluding to that alternative, Kerry declared: "We would be fools to walk away."

___

Margaret Childs in Vienna and Associated Press writer Nasser Karimi in Tehran contributed to this report.