Kerry Meets Iranian Foreign Minister Zarif

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WATCH: Iranian President Hassan Rohani said that he was committed to moderation and that his government was looking for a mutually acceptable solution in nuclear talks. (Reuters)

U.S. Secretary of State John Kerry has held a one-on-one meeting with Iran's foreign minister and says he is pleased with a new positive tone from Iran on talks over its disputed nuclear program.

The brief encounter between Kerry and Foreign Minister Mohammad Javad Zarif at UN headquarters in New York on September 26 was one of the highest-level meetings between the United States and Iran since the 1979 Iranian Revolution.

Afterward, Kerry was upbeat, but cautious.

"I have just met with him [Zarif] now on a side meeting, which we took a moment to explore a little further the possibilities of how to proceed based on what President [Barack] Obama laid out in his speech to the General Assembly earlier this week," Kerry said.

The two met after taking part in a meeting between Iran and the five permanent members of the UN Security Council plus Germany on Tehran's disputed nuclear program.

Kerry said those talks had been constructive, a sentiment shared by Zarif.

"Discussions were very substantive, business-like," Zarif told reporters.

"We hope to be able to make progress towards resolving this issue in a timely fashion based on respecting the rights of the Iranian people to nuclear technology for peaceful purposes including enrichment, and at the same time making sure that there is not concern at the international level that Iran's nuclear program is anything but peaceful."

EU foreign-policy chief Catherine Ashton was upbeat as well.

"It was a substantial meeting; good atmosphere; energetic. We had a discussion about how we would go forward with an ambitious time frame to see whether we can make progress quickly. I'm pleased that we agreed to meet in Geneva on October 15 and 16 to pursue the agenda to carry on from today's meeting and to hopefully move this process forward," Ashton said.

Concrete Step

In an interview aired later on U.S. TV, Kerry said the United States would not lift sanctions on Iran until it showed it is not pursuing a nuclear-weapons capability.

Kerry said one concrete step Iran could take to show it was serious about not seeking nuclear arms would be to open up its Fordow uranium-enrichment facility near the religious city of Qom to UN inspectors.

Tehran has repeatedly denied Western charges it is secretly developing nuclear weapons.

Iranian President Hassan Rohani and Zarif -- both in New York this week to attend the UN General Assembly -- have said their government is eager to secure a deal with the West on its nuclear program in order to ease crippling international sanctions.

Earlier on September 26 at the UN, Rohani called for a world without nuclear weapons, and urged Israel to sign up to the Nuclear Nonproliferation Treaty.

With reporting by AP and Reuters