A level of enrichment greater than 20 percent is the very minimum needed to make a crude nuclear device, scientists say, although most nuclear bombs use the heavy metal enriched to 90 percent.

Western powers believe Iran is attempting to build an atomic weapon, but Tehran insists its program has entirely peaceful purposes and has decried international pressure as unacceptable interference in its sovereign affairs.

Iran’s chief negotiator, Saeed Jalili, said on Wednesday the talks had seen a modicum of progress, and proposals by the world powers were “closer to Iran’s position.”

“Their proposals seem more realistic and positive,” he said.

And despite the lack of a breakthrough at the first talks between the sides since last June’s meeting in Moscow, Russia’s chief negotiator at the two-day talks, Deputy Foreign Minister Sergei Ryabkov, described the negotiations as “extremely useful.”

“A number of aspects to the resolution of the issue and means of increasing trust were considered,” he said, without elaborating.

The two sides agreed to meet again at expert level in Istanbul on March 17-18 and to hold high-level talks in Kazakhstan on April 5-6.

Iranian state television earlier said the atmosphere at the talks was "very serious." The country’s supreme leader, Ayatollah Ali Khamenei, reiterated in comments two weeks before the Almaty negotiations that Iran was not seeking an “Un-Islamic atomic weapon,” but said there was nothing the United States could do if it were.

US suspicions that Iran wants to build an atomic weapon date back decades. A February 1992 report by the US House of Representatives suggested that Tehran would possess “operational” nuclear weapons by April of that year.

Both Israel, widely believed to possess undeclared nuclear weapons, and the United States have refused to rule out an attack on Iran over its disputed atomic program.

New US Secretary of State, John Kerry, said on Monday the opportunity for negotiations could not “remain open forever.”