Iran said it was in touch with world powers to reopen talks soon on its nuclear programme, but Washington and the European Union denied this and said they were still waiting for Tehran to show it was ready for serious negotiations.
A year after the last talks fell apart, confrontation is brewing over Tehran's nuclear work, which the United States and other countries say is focused on developing atomic weapons. Iran dismisses the accusation.
The EU is preparing to intensify sanctions against Iran with an embargo on its economically vital oil exports. EU diplomats said on Wednesday member governments had also agreed in principle to freeze the assets of Iran's central bank, but had yet to agree how to protect non-oil trade from sanctions.
Iran has threatened to block the Strait of Hormuz, used for a third of the world's seaborne oil trade, if it cannot sell its own crude, fanning fears of a descent into war in the Gulf that could inflame the Middle East.
Iranian politicians said U.S. President Barack Obama had expressed readiness to negotiate in a letter to Tehran, a step that might relieve tensions behind recent oil price spikes.
"Negotiations are going on about venue and date. We would like to have these negotiations," Iranian Foreign Minister Ali Akbar Salehi told reporters during a visit to Turkey.
"Most probably, I am not sure yet, the venue will be Istanbul. The day is not yet settled, but it will be soon."
Washington denied there were any new discussions underway about resuming talks, but declined to comment on whether Obama had sent a letter to Tehran.
"There are no current talks about talks," State Department spokeswoman Victoria Nuland said on Wednesday.
"What we are doing, as we have said, is making clear to the Iranians that if they are serious about coming back to a conversation, where they talk openly about their nuclear programme, and if they are prepared to come clean with the international community, that we are open to that," Nuland said at a media briefing.
The United States is pushing countries to reduce the volume of Iranian oil they buy in line with a new sanctions law Obama signed on Dec. 31 that targets Tehran's ability to sell crude oil.
The State Department denial was echoed by a spokesman for EU foreign policy chief Catherine Ashton, who represents the six world powers trying to engage with Iran.
"There are no negotiations under way on new talks," he said in Brussels. "We are still waiting for Iran to respond to the substantive proposals the High Representative (Ashton) made in her letter from October."
British Foreign Secretary William Hague said Iran had to be ready for serious negotiations. "It is significant that when we are discussing additional sanctions in the European Union an offer of negotiations emerges from Iran," he said.
"We will not be deterred from imposing additional sanctions simply by the suggestion there may be negotiations. We want to see actual negotiations," he told a news conference in Brazil.
Tehran denies wanting nuclear bombs, saying its enrichment work is for power generation and medical applications.
The last talks between Iran and the permanent members of the U.N. Security Council - the United States, Britain, France, Russia and China - along with Germany stalled in Istanbul a year ago, with the parties unable to agree even on an agenda.
The six countries have also failed to agree on a common line in their relations with Iran.
Chinese Premier Wen Jiabao defended his country's extensive oil trade with Iran against Western sanctions pressure in comments published on Thursday. Even so, he said Beijing firmly opposes any efforts by Tehran to acquire nuclear weapons.
Russian Foreign Minister Sergei Lavrov said a last-ditch military option mooted by the United States and Israel would ignite a disastrous, widespread Middle East war. Russia has also criticised the new sanctions.