Geneva -- Iran and world powers including, for the first time, the United States, failed to find a way toward full negotiations in Geneva on Saturday, as Tehran's representatives did not agree to the precondition of suspending uranium enrichment.

Speaking at a news conference after talks with Iranian negotiator Saeid Jalili, EU chief diplomat Solana said that "the most important question" in the dispute with Iran remained unanswered.

On behalf of the five United Nations Security Council veto powers and Germany, Solana met with Jalili to talk about future economic, political and nuclear energy cooperation, once Tehran halts its nuclear activities.

The meeting had the goal of achieving full negotiations by squaring the world powers' precondition of nuclear suspension with Iran's insistence on its right to civilian nuclear energy.

"We have not gotten an answer," Solana told reporters. "We have talked frankly."

He suggested that talks with Iran would continue within weeks, expressing his hope that Tehran would reply within 14 days.

After the meeting, the United States urged Tehran to respond soon to a package of incentives offered by world powers in exchange for an Iranian pledge to suspend uranium enrichment. State Department spokesman Sean McCormack said in Washington that the US would support additional UN Security Council sanctions if Iran failed to accept the offer.

"We hope the Iranian people understand that their leaders need to make a choice between cooperation, which would bring benefits to all, and confrontation, which can only led to further isolation," McCormack said.

Seeking to break the diplomatic stalemate over Iran's nuclear activities, the United States broke with past policy and sent a senior envoy, Under Secretary of State William Burns, to the talks to demonstrate that Washington was committed to a diplomatic solution.

Burns was the highest-ranking US diplomat in three decades to meet with an Iranian envoy. US Secretary of State Condoleezza Rice has made it clear that despite this signal to Tehran, the US still demands that Iran halt uranium enrichment before full negotiations can begin.

During the meeting, Burns reiterated the US position but did not negotiate with the Iranians or hold one-on-one talks with Jalili.

Solana and Jalili both said the talks had been "constructive."

Britain, France, Germany, the United States, Russia and China are offering Iran a "freeze-for-freeze" approach. In a pre-negotiation phase, Tehran would not expand its Natanz enrichment facility, while the six powers would not press for additional Security Council sanctions.

After confidence between the two sides has been established in the first phase, Iran would halt its uranium enrichment, and comprehensive talks could start on the world powers' offer of cooperation and a similar package put forward by Iran.

Jalili said that both sides had talked about the freeze-for-freeze phase for "many, many hours." He said it was more important to find a "constructive approach ... to address our common concerns."

After the talks, an unnamed Iranian official in Tehran told Iran's ISNA news agency that Jalili "has proposed to keep the 6+1 model as this model would be more effective to reach a settlement for this (nuclear dispute) issue as well as regional matters."

McCormack said the US role in the meeting "strengthened the position" of the six powers in their commitment to pursue "further disincentives" -- including sanctions -- "should Iran not choose the path of cooperation."

At the Geneva meeting, Iran presented a paper that focuses on future cooperation without addressing the six nations' demands, a Western diplomat said.

Representatives from Britain, France, Russia and China and Germany also attended the meeting in Geneva. The six countries are concerned that Iran could one day use its civilian nuclear programme to build nuclear weapons, an allegation that Tehran strongly denies.

Jalili compared the ongoing diplomacy to carpet weaving that "moves ahead in millimetres."

"And again, to some extent it is similar to Iranian carpets because it is a very precise work, it's in certain cases a very beautiful endeavor, and hopefully the end result, the final product, would be beautiful to behold," he said.

World powers have tried to stop Tehran's nuclear programme ever since it became clear in 2002 that Iran had hidden its activities from IAEA inspectors for 18 years.

DPA