Iran said on Tuesday it was ready for "serious negotiations" on its nuclear programme, insisting it was not trying simply to buy time to develop weapons. A semi-official news agency said the government was unwilling to abandon uranium enrichment - the key US demand.
The statements came as the government delivered its written response to a package of incentives offered by the United States and five other world powers if Iran rolls back on its nuclear program - and punishments if it does not.
The US ambassador to the United Nations, John Bolton, said Washington will "study the Iranian response carefully" but was prepared to move forward with sanctions against Tehran if it was not positive.
The White House held off commenting until it had studied the text. The European Union's foreign policy chief, Javier Solana, said the document was "extensive" and required "a detailed and careful analysis."
UN Undersecretary-General Ibrahim Gambari, meanwhile, said Secretary-General Kofi Annan had urged the Iranians to be forthcoming and respond positively.
"It's a serious situation, but it has to be handled with great care and, of course, the Iranians have to meet their obligations under the NPT (Nuclear Proliferation Treaty), but we'll wait for them," he said.