Spokesman Tony 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 two are expected to meet in the coming days, although a precise time and venue have yet to be set.
"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.
"There are differing voices coming out of Iran. This is the one that we and our EU partners have said would be the appropriate track for responding, and therefore that’s the track that we would expect to hear through."
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.
The United States has insisted that Iran has "weeks, not months" to reply.
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.
"I am convinced that dialogue and not isolation of one or another state is what leads to resolution of crises," Putin said.
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.