Iran must give an answer within weeks to an international offer of incentives to suspend its nuclear programme, the US president has said.
Iran's nuclear programme was kept secret for years
George W Bush said Iran had to respond to the offer in "weeks, not months", a first clear time frame since the offer.
Major powers want Iran to curb uranium enrichment in return for relaxing restrictions on trade and agriculture.
Separately, Austria's EU president said on Friday that Iran had to respond by the time of the G8 summit in July.
Speaking alongside the Danish prime minister at his retreat in Camp David, Maryland, Mr Bush repeated that Iran would face consequences if it failed to respond.
"We've given the Iranians a limited period of time - you know, weeks, not months - to digest a proposal to move forward.
"And if they choose not to verifiably suspend their programme, then there will be action taken within the UN Security Council," he said.
The international proposal to Iran, designed to break the diplomatic impasse over Tehran's nuclear programme, was first delivered last week.
Western diplomats have signalled their satisfaction with Iran's initial response to the plans, which may allow Tehran to enrich uranium at some point in the future.
Russia and Germany had said they expected Iran to respond to the proposal by the end of June.
Iran allowed to buy spare parts for civilian aircraft made by US manufacturers
Restrictions lifted on the use of US technology in agriculture
Provision of light water nuclear reactors and enriched fuel
Support for Iranian membership of World Trade Organisation
From Western diplomatic sources
But Austrian Chancellor Wolfgang Schuessel implicitly backed Mr Bush's time frame on Friday, saying that Iran would have until the G8 nations met in July to consider the plans.
Since the original plans were delivered to Tehran in secret by EU foreign policy chief Javier Solana, reports have suggested that Iran has begun a fresh round of uranium enrichment.
The US and other powers suspect Iran is aiming to develop a nuclear weapons capability, but Tehran insists is programme is only aimed at developing nuclear energy.