Later, pressed on whether he had meant to signal that Iran's private responses were more positive than its public statements, Snow replied: "I make absolutely no attempt to characterize -- good, bad, or indifferent."

Snow said the United States would only recognise a formal answer given by Iran's senior nuclear negotiator, Ali Larijani, to European Union foreign policy chief Javier Solana.

"The position has always been the same, which is: When Mr Larijani communicates with Javier Solana, that is how we expect to have an answer to the proposal," Snow told reporters.

Diplomats said hopes had dimmed that the two would meet this week despite an informal deadline on Thursday for Iran to declare to world powers if it would suspend uranium enrichment.

Iranian President Mahmoud Ahmadinejad has said a reply to the proposal will be given in late August. In contrast, the major powers are calling for a reply before the end of June in time for a summit of the G8 group of industrialised nations in Russia.

UN Secretary General Kofi Annan on Tuesday also urged Iran to expedite its response to the offer of incentives.

Annan conveyed his view "that Iran should speed up its response to the proposals" in a meeting with Iranian Foreign Minister Manouchehr Mottaki at UN headquarters in New York, a UN statement said.

But President Vladimir Putin said, in a thinly veiled reference to US-led pressure on Iran, that Russia will not join any ultimatums over the problem of nuclear proliferation.

"We do not intend to join any sort of ultimatum, which only pushes the situation into a dead end, striking a blow against the authority of the UN Security Council," Putin told Russian diplomats in Moscow in the presence of journalists.

Russia, a key economic ally of Iran, has consistently resisted Western pressure in the current international impasse over US and European claims that Tehran is using a Russia-backed civilian nuclear programme to mask a secret bomb-making project.

The United States has not ruled out seeking UN sanctions or even military action should Iran refuse the talks.