"I believe it is possible to reach an understanding for an agreement before we close these negotiations (Friday) evening," Iran's foreign minister and chief nuclear negotiator, Javad Zarif, told CNN's Christiane Amanpour.

Zarif's optimistic comments came after a day of meetings with diplomatic and technical teams from United States, France, Britain, Russia, China and non-nuclear-armed Germany -- referred to as the P5+1 or EU3+3 -- and Iran.

But he would not detail what such an agreement could include.

"We are at a very sensitive stage of negitations, and it is best if these negotiations are done at the negotiating table rather than on live television," he said. "But I can tell you that we are prepared to address some of the most immediate concerns that have been raised, and we expect reciprocally our concerns to be met by the P5+1."

A State Department official described Thursday's talks as "substantive and serious."

A bilateral meeting -- part of an apparent effort on each side to mend fences -- lasted about an hour on the sidelines of broader talks under way in Geneva.

The U.S. delegation was led by Under Secretary of State Wendy Sherman. The Iranian delegation was led by Deputy Foreign Minister Abbas Araqchi.

Uranium enrichment is the key issue under discussion at this week's negotiations with Iran over the future of its controversial nuclear program, a European Union spokesman said Thursday.

But the top Iranian delegate was reported by Iranian state media as saying Iran would not stop enriching uranium and describing that demand -- backed by United Nations resolutions -- as a "red line."

Iran talks: Do we want a deal or a war?

The apparently contrasting statements underlined the complex nature of the talks between Iran and the five permanent members of the U.N. Security Council, which are also nuclear-armed nations.

"The main issue is getting to the root of the problem, which is the enrichment issue and all things that lead from that," Michael Mann, spokesman for EU foreign policy chief Catherine Ashton, said on the sidelines of negotiations.

Iran's supreme leader 'not optimistic' about nuclear talks

Moments later, the Iranian state-run Fars News Agency tweeted a quote from Araqchi: "Enrichment is our red line and its suspension is unacceptable."

When CNN asked Araqchi about the issue of uranium enrichment, he declined to answer.

"The talks are extremely complex and are now going into a serious phase," Mann said. "We want to focus on substance and hope there will be concrete progress over the next couple of days."

The United States and its partners have accused Iran of diverting resources from its civilian nuclear program to try to develop a nuclear bomb. Iran has repeatedly denied those assertions and says it has no desire to build nuclear weapons.

U.S. official: We think Iran wants a nuclear deal -- and fast

Resolutions and sanctions passed by the United Nations in 2006 called on Iran to halt all enrichment activities and clarify that its nuclear facilities were being used for peaceful purposes only.

In its August report, the U.N. nuclear watchdog, the International Atomic Energy Agency, estimates that since declaring its nuclear program, Iran has processed 10 metric tons of uranium to 5% purity, the level used for nuclear power plants.

The IAEA estimates that Iran has a stockpile of 185 kilograms of uranium at 20% purity. Weapons experts warn that this uranium could be further refined for use in a nuclear warhead. Although the experts suggest that amount would not be enough for a single warhead, the IAEA has warned that it believes Iran's nuclear program could have "possible military dimensions."

Ahead of the latest round of talks, on Wednesday night, a senior U.S. administration official said the United States would look for a "first step" agreement from Iran to "halt its nuclear program and even roll back parts."

Mann described the negotiating stance in broader terms.

"The goal of the EU3+3 is very clear, and that's to achieve assurances from Iran about the purely peaceful nature of the nuclear program. And we can do that if there's agreement from the Iranian side," he said.

He also underlined that the six nations have a solidly united negotiating position.

"The P5+1, or EU3+3, has remained united and will remain united. We know what the issues are. There are resolutions concerning enrichment that have to be respected. Iran must prove verifiably that it is only interested in peaceful nuclear power," he said.

While revealing few details, the Iranians and the Americans are fueling optimism of rapid progress to a first-step agreement. The sides have agreed in principle that, after reaching a first-step deal, they will discuss a final agreement.

In exchange for concessions by Iran on its nuclear program, a senior U.S. administration official said the United States would give Iran some sanctions relief.

Economic analysts estimate that sanctions imposed by the United Nations, the United States and the European Union have slashed Iran's key oil output to below 1 million barrels per day, less than half 2011 levels, and are driving inflation and unemployment inside Iran as well as hampering Tehran's ability to repatriate foreign currency earnings from its oil exports.

Since at least 1974, the U.N. Security Council has called for a nuclear-free zone in the Middle East. Israel is believed to be the only Middle Eastern nation with nuclear weapons, although it has never publicly declared this nor signed the nuclear Non-Proliferation Treaty, which came into effect in 1970.

Mann said there would be no linkage between Iran's nuclear program and that of Israel.

"We're here to talk about Iran," he said.

Source:CNN News International