The offer, to defer new economic sanctions in return for Iran suspending its uranium enrichment programme, was made two weeks ago when US and Iranian diplomats discussed the six-year nuclear crisis for the first time.
It had followed weeks of sabre rattling in which Israel rehearsed bomb attacks on Iran which in turn tested long range missiles that could strike Israel.
"We have not had any discussion or agreement of the so-called timeline," said Ali Asghar Soltaniyeh, Iran's ambassador to the United Nations nuclear watchdog.
But his brusque rejection of the deadline imposed by the US and the other countries, including Britain, which made the offer, is unlikely to deter international pressure.
"We want and we expect a response this weekend," a US State Department spokesman said. "They were given two weeks. The two weeks is up this weekend."
European diplomats have given Iran a little more leeway, but insist that it answer their proposals within a few days.
"One should not focus too much on Saturday," an EU official said. "If it's not Saturday but next week, we'll not make a big fuss about it. What matters is to get a clear answer quickly, in the very coming days."
However, despite Iran's prevarication, there are signs of increasing internal debate over whether it should, eventually, accept some sort of compromise.
While the president Mahmoud Ahmadinejad launched a new anti-Western diatribe on Friday, saying the US was "in decline", his powerful political opponents are pushing for more measured language.
Akbar Hashemi-Rafsanjani, a former president who retains influence in top policy circles, rejected the notion of a deadline but said Iran must keep talking.
"The plan now is to negotiate," he said at Friday prayers one week ago. "What are all ultimatums that you are issuing, that you have to take in two weeks? The plan is to talk. Iran is ready. It will come there and talk."
While Western countries accuse Iran of trying to build a nuclear bomb and hiding key parts of its plans, Iran has always said its programme is purely civilian and that it has not breached international rules.
Several years of negotiations have failed to stop Iran pursuing the process - nor have two-and-a-half years of sanctions and mounting US pressure.
If Iran continues to reject a suspension, Western countries still say they will negotiate while pushing for sanctions. But with Russia and China only reluctantly agreeing to tougher action against Iran, these could take months to be approved.
© Copyright of Telegraph Media Group Limited 2008