Iran said its reply to the powers' nuclear incentives offer contained ideas that would allow serious talks about its standoff to start straightaway.
But there was no sign on Tuesday Tehran had agreed to a key UN Security Council demand that it freeze uranium enrichment by August 31 or face the prospect of sanctions. Iran has called the deadline meaningless.
One EU diplomat said Iran had ruled out halting enrichment before talks "but indicated that it might be open to accept suspension in the course of negotiations".
Other diplomats declined to confirm Iran had shown flexibility on enrichment.
The five permanent Security Council members - Britain, China, France, Russia and the US, plus Germany, which offered Iran the incentives to stop enrichment - were tight-lipped on Wednesday on their response.
Javier Solana, the European Union foreign policy chief, would only say that Iran's answer was "extensive and therefore requires a detailed and careful analysis".
A White House spokesman said on Tuesday that George Bush had yet to examine the Iranian reply.
Iran had said its reply to the package of economic, security and nuclear incentives would be "multi-dimensional", suggesting no simple "yes" or "no".
Designed to divide
Iran's answer was likely to be designed to divide Security Council members Russia and China, both key trade partners of Tehran lukewarm about sanctions, from the US, France and Britain which have backed tougher measures.
Jon Wolfsthal, of the Centre for Strategic and International Studies, said: "They are betting that they can splinter the coalition and that they can carve off one or two members of the Security Council in supporting something less than suspension.
"The question is can the US convince these countries [China and Russia] to stay on board?"