During a visit to Washington, Israel's deputy Prime Minister Shaul Mofaz warned meanwhile that Iran was playing for time in dealing with the world community and was heading for a "major breakthrough" in its nuclear capability.
The US State Department had been vague about the deadline for Iran to reply to the offer but narrowed it down on Friday.
"We want and we expect a response this weekend," the State Department's acting spokesman Gonzalo Gallegos told reporters. "They were given two weeks. The two weeks is up this weekend."
Secretary of State Condoleezza Rice had given Iran two weeks to come up with a "serious" reply after an international meeting in Geneva on July 19 which saw Tehran broadly accused of stonewalling.
White House spokeswoman Dana Perino warned: "Negative consequences await if they don't have a positive response to our very generous incentives package, and that would possibly come in the form of sanctions."
Perino also said it was difficult to discern Iranian intentions, calling them "a little bit unpredictable" as she spoke to reporters in Kennebunkport, Maine.
"The Iranians sent mixed messages this week and it's very hard to tell what the bottom line is," Perino said.
Perino said the United States would coordinate any action with its partners in the P5-plus-1, or the permanent UN Security Council members — the United States, Russia, China, Britain and France — plus Germany.
But Washington's European Union partners in the drive to stop Iran's uranium enrichment program stopped short of insisting on a strict deadline and believe that a reply within a few days would suffice, an EU diplomat said.
After meeting Iran's negotiator in Geneva, Solana asked for a response in two weeks, but "if it's in 16 days instead of 14 it's not a problem. We are not obsessed with a date," the diplomat said on condition of anonymity.
Iranian Foreign Minister Manouchehr Mottaki said Thursday that there was no deadline and that his country had already replied.
Iranian President Mahmoud Ahmadinejad also said on Friday that his country would use force against its "enemies" to defend its nuclear drive, state television reported.
The United States has taken a more conciliatory approach lately.
In a policy shift, Washington sent top diplomat William Burns to the talks in Geneva to encourage those in Iran who want to cooperate with the West in order to ease the economic and financial fallout from UN sanctions.
Washington had until then refused to sit down with Iran until it suspended uranium enrichment.
Iran expert Gary Sick said Washington had learned that its past desire to isolate Tehran with increasingly stiff sanctions had failed to stop Iran from enriching uranium.
The West charges Iran with trying to build an atomic bomb. Iran denies the charges and says the program is for peaceful nuclear energy.
The P5-plus-1 has offered Iran benefits in civil nuclear energy, trade, finance, agriculture and high technology if it freezes uranium enrichment.
If Iran accepts the package, there would be pre-negotiations during which Tehran would add no more uranium-enriching centrifuges and, in return, face no further sanctions.
European Union foreign policy chief Javier Solana delivered the incentives package to Tehran in June.
