The U.S. and its European allies said Thursday they were pleased by a new tone and a significant shift in attitude from Iran in talks aimed at resolving the impasse over the country's disputed nuclear activities. Iran said it was eager to dispel suspicions that it is trying to develop a nuclear weapon and to get punishing international sanctions lifted as fast as possible.

Iranian Foreign Minister Mohammad Javad Zarif, who also had an unexpected one-on-one meeting with U.S. Secretary of State John Kerry, said six world powers and Iran had agreed to fast-track nuclear negotiations with the hope of reaching a deal within a year.

Iran, the five permanent members of the U.N. Security Council and Germany also agreed to hold a new round of substantive nuclear negotiations on Oct. 15-16 in Geneva.

“We agreed to jump-start the process so that we could move forward with a view to agreeing first on the parameters of the end game ... and move toward finalizing it hopefully within a year's time,” Zarif said after the talks ended. “I thought I was too ambitious, bordering on naiveteuro. But I saw that some of my colleagues were even more ambitious and wanted to do it faster.”

Kerry said he was struck by a “very different tone” from Tehran after their sessions, which marked the highest-level direct contact between the United States and Iran in six years.  But, like his European colleagues, he stressed that a single meeting was not enough to assuage international concerns that Iran is seeking to develop nuclear weapons under cover of a civilian atomic energy program.

“Needless to say, one meeting and a change in tone, that was welcome, does not answer those questions,” Kerry told reporters. “All of us were pleased that the foreign minister came today and that he did put some possibilities on the table.”

He said they agree to continue the process and try to find concrete ways to answer the questions that people have about Iran's nuclear activities.

Zarif and Kerry sat next to each other at a U-shaped table during the group talks. It was the highest-level direct contact between the United States and Iran in six years. EU foreign policy chief Catherine Ashton suggested the two men had shaken hands and been cordial with each other.

She also said the parties had agreed to “go forward with an ambitious timeframe.”

Zarif said the meetings were “very constructive” and “very substantive.”

“We hope to be able to make progress to solve this issue in a timely fashion (and) to make sure (there is) no concern that Iran's program is anything but peaceful,” he said. “I am satisfied with this first step,” he added. “Now we have to see whether we can match our positive words with serious deeds so we can move forward.”

He said the end result would have to include “a total lifting” of the international sanctions that have devastated Iran's economy.

British Foreign Secretary William Hague said there had been a “big improvement in the tone and spirit” from Iran compared with the previous government under Mahmoud Ahmadinejad.

German Foreign Minister Guido Westerwelle said the meeting had taken place in a “completely different tone, atmosphere and spirit” than what the group was used to and that a “window of opportunity has opened” for a peaceful resolution of the situation. He too insisted that Iran's words would have to be matched by actions.

“Words are not enough,” he said. “Actions and tangible results are what counts. The devil is in the detail, so it is now important that we have substantial and serious negotiations very soon.”

Iranian President Hassan Rouhani and Zarif, both in New York this week to attend the U.N. General Assembly, have said they are anxious to clinch an agreement quickly that could bring relief from sanctions that have slashed the country's vital oil exports, restricted its international bank transfers, devalued the currency and sent inflation surging.

Encouraged by signs that Rouhani will adopt a more moderate stance than Ahmadinejad, but skeptical that the country's all-powerful supreme leader will allow a change in course, President Barack Obama has directed Kerry to lead a new outreach and explore possibilities for resolving the long-standing dispute.

Rouhani has come across as a more moderate face of the hard-line clerical regime in Tehran and his pronouncements at the U.N. have raised guarded hopes that progress might be possible. But they have also served as a reminder that the path to that progress will not be quick or easy.

In his speech to world leaders at the U.N. on Tuesday, he repeated Iran's long-standing demand that any nuclear agreement must recognize the country's right under international treaties to continue enriching uranium.

The U.S. and its allies have long demanded a halt to enrichment, fearing Tehran could secretly build nuclear warheads. They have imposed sanctions over Iran's refusal to halt enrichment. Uranium enriched to low levels can be used as fuel for nuclear energy but at higher levels, it can be used to make a nuclear weapon.

Rouhani also insisted that any deal be contingent on all other nations declaring their nuclear programs, too, are solely for peaceful purposes _ alluding to the U.S. and Israel.

Those conditions underscored that there is still a large chasm to be bridged in negotiations.

Rouhani has made a series of appearances and speeches since arriving in New York and has held bilateral negotiations with France, Turkey and Japan among others.

On Thursday, he called for worldwide disarmament of nuclear weapons as “our highest priority.”

“No nation should possess nuclear weapons, since there are no right hands for these wrong weapons,” he told the first-ever meeting of a U.N. forum on nuclear disarmament. He was speaking on behalf of the Non-Aligned Movement, an organization of mostly developing countries.

He repeated the organization's long-standing demand that Israel join the international treaty banning the spread of nuclear weapons.

Israel, which has repeatedly accused Iran of aspiring to build a nuclear bomb, is the only Mideast state that has not signed the landmark 1979 Non-Proliferation Treaty.

Rouhani appears to be trying to tone down Ahmadinejad's caustic rhetoric against Israel _ a point of friction in relations with the U.S. But Israel is not biting and reacted angrily to his latest remarks.

“Iran's new president is playing an old and familiar game by trying to deflect attention from Iran's nuclear weapons program,” said Intelligence and International Affairs Minister Yuval Steinitz. “The problem of the NPT in the Middle East is not with those countries which have not signed the NPT, but countries like Iran, Iraq, Libya and Syria which have signed the treaty and brazenly violated it,” he added.

“Unlike Iran, Israel has never threatened the destruction of another country,” he said.

Iran watchers say Rouhani may have limited time to reach a settlement _ possibly a year or less _ before Khamenei decides negotiations are fruitless. That may explain why Zarif has call to reach a deal in shortest timeframe possible.

Already, Iran's powerful Revolutionary Guard force has grown increasingly uneasy over Rouhani's outreach to the West as well as his apparent backing from Khamenei, who has told the Guard to steer clear of politics.

The Guard has warned Rouhani about moving too fast on his overtures. (AP)