Iran and world powers remain far apart over Tehran’s nuclear program and a “difficult road” lies ahead, a senior Iranian negotiator said on Thursday, according to Reuters.

Iran’s Deputy Foreign Minister Abbas Araqchi spoke after a day of talks in Vienna with the three European members of the group of six major powers that is seeking to negotiate an end to a decade-old nuclear dispute with Iran.

The discussions between Iran and Britain, France and Germany took place less than a week after Iran and the United States held a bilateral meeting in Geneva.

Asked how big the differences were, Araqchi told reporters, “Still big.” He added, “We are always optimistic … but we have a difficult road to go.”

The six powers, also including Russia and China, will hold their first full negotiating round with Iran since July on September 18 in New York, seeking to narrow gaps over the future size of Iran’s uranium enrichment infrastructure and other issues.

Iran and the powers failed to meet a July 20 target date for an agreement and have extended the deadline to November 24.

Iran denies Western allegations that it is refining uranium to develop the capability to assemble nuclear weapons, saying it is doing it to help generate electricity.

Araqchi said the talks with the European officials had been useful ahead of what he called next week’s real negotiations.

“It was just some consultations and exchange of views trying to bridge the gaps between the two sides,” he said, according to Reuters. “We hope to make progress next week in New York.”

Diplomats say the main stumbling block is disagreement on how many centrifuges Iran should be allowed to keep to refine uranium, with Tehran rejecting demands to significantly reduce the number below the more than 19,000 it has now installed, of which roughly half are operating.

Iran has been taking an increasingly aggressive line in demanding its “right” to enrich uranium, with Iranian Supreme Leader Ayatollah Ali Khamenei recently saying Iran “needs” 19 times more nuclear centrifuges than the amount being offered by world powers.

Araqchi’s pessimistic tone regarding the talks is in contrast to that of the European Union’s foreign policy chief, Catherine Ashton, who said this week that while Iran is far apart from the six world powers, that gap could be narrowed.