Javier Solana on his hopes for an answer from Iran
Iran must decide between confrontation and co-operation in the dispute over its nuclear plans, the US has warned.
At talks in Geneva, envoys from the US, EU and UN asked Iran to suspend its uranium enrichment in return for a pledge not to introduce new sanctions.
Iran gave no guarantees it would halt its activities, so the diplomats gave Tehran two weeks to provide an answer.
The meeting was the first time US and Iranian officials have held face-to-face talks on the nuclear issue.
Senior US official William Burns was present at the Geneva talks - although he made no public comment.
Instead, state department spokesman Sean McCormack issued a strongly-worded statement in Washington.
"We hope the Iranian people understand that their leaders need to make a choice between co-operation, which would bring benefits to all, and confrontation, which can only lead to further isolation," he said.
Mr McCormack added that Mr Burns had delivered a "clear simple message" that Washington was "serious" about the incentives package but that it would only negotiate with Iran if it upheld its side of the deal.
Diplomats had hoped that Iran would respond to a so-called "freeze-for-freeze" offer, under which a freeze of Iran's uranium enrichment programme at its current levels would be matched by a Western pledge not to strengthen sanctions on Tehran.
"It was a constructive meeting, but still we didn't get the answer to our questions," EU envoy Javier Solana told reporters.
Iran suspends its nuclear activities including the installation of any new centrifuges
At same time the six world powers refrain from any new Security Council resolution on sanctions
Talks can then start on long-term deal on recognising Iran's right to develop nuclear energy for civilian purposes, and lifting of sanctions
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