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.
"We hope very much we get the answer and we hope it will be done in a couple of weeks," he said.
Mr Solana said he had agreed with Iran's chief negotiator, Saeed Jalili, to speak again either by telephone or personally in two weeks.
The BBC's Jon Leyne in Tehran says Iran is interested in the offer but it is unclear whether there are divisions in the leadership or the Iranians are playing for time.
Mr Jalili said he had put forward many positive ideas and he urged Western powers not turn away from negotiations.
"This package we have proposed contains a number of possibilities. In a nutshell, it is a new opportunity which should not be lost."
But doubt was cast over the value of the talks, after a member of the Iranian delegation said there was "no chance" of a freeze on the uranium-enrichment programme.
Iran says its nuclear facilities are designed to meet its energy needs, denying that it has a weapons programme.
But Tehran's continued activity is defying UN Security Council demands to halt enrichment.
In addition to the EU, Iranian and US envoys, the talks in Geneva's city hall were attended by representatives from Britain, China, France, Germany and Russia.
The US and Iran have had no diplomatic relations since the 1979 Iranian Revolution and the taking of hostages at the US embassy in Tehran.
Formal contact between the two countries has been extremely limited, though last year they met at ambassadorial level to discuss security in Iraq.
The meeting came after weeks of rising tensions in the Middle East.
The Iranians test-fired missiles last week, and a series of threats and counter-threats between Iran and Israel has been watched nervously in the West.