Speaking in Vienna after talks with European Union leaders on Wednesday, George W Bush said Iran's plan to reply by late August to an offer of incentives to halt nuclear work was "an awful long time for a reasonable answer".
He said: "It shouldn't take the Iranians that long to analyse what is a reasonable deal.
Referring to Britain, France, Germany, Russia and China, Bush said: "I said weeks, not months. I believe that's what the other partners [say too]."
Mahmoud Ahmadinejad, the Iranian president, had announced earlier on Wednesday that Tehran would respond to the proposal by August 22, the first specific date given by Iran since the incentives package was delivered on June 6.
The major powers have set an informal deadline of mid-July, when a Group of Eight industrialised nations summit is planned.
Western diplomats have said Iran's hesitation may be a stalling manoeuvre aimed at buying time to expand its nuclear fuel enrichment programme and make it a fait accompli.
Diplomats and analysts in Tehran said the delay is more a sign of debate within a complex Iranian power structure over how to respond.
Iran says its drive to enrich uranium is solely to provide electricity for its economy. The West suspects the programme is a disguised atomic bomb project.