Six major world powers and Iran are to hold fresh talks on Tehran's nuclear programme, the EU has said.
EU foreign policy head Catherine Ashton said she had replied to a letter from Iran on behalf of the five permanent UN Security Council members plus Germany.
Iranian negotiator Saeed Jalili sent the letter last month proposing talks. No date or venue has been set.
The move comes amid fresh speculation of a pre-emptive military strike against Iran's nuclear facilities.
Iran insists there is no military element to its programme but Western powers fear it is constructing nuclear weapons.
US President Barack Obama told a news conference in Washington DC that he believed there was still a "window of opportunity" to use diplomacy to resolve the standoff with Tehran.
He said there was no need to make a choice in the next fews weeks or months on whether to use military means to stop Iran building nuclear weapons.
The statement from Baroness Ashton said the EU hoped that Iran would "now enter into a sustained process of constructive dialogue which will deliver real progress in resolving the international community's long-standing concerns on its nuclear programme."
It added: "Our overall goal remains a comprehensive negotiated, long-term solution which restores international confidence in the exclusively peaceful nature of Iran's nuclear programme."
UK Foreign Secretary William Hague said in a statement that Iran had to "convince the international community that its nuclear programme is exclusively peaceful".
"Until those actions are taken we will not ease the international pressure on Iran."
Iran had earlier said it was prepared under certain conditions to grant inspectors from the UN's International Atomic Energy Agency (IAEA) access to the key military site of Parchin.
The complex, south of Tehran, is dedicated to the research, and the development and production of ammunition, rockets and explosives.
IAEA inspectors wanted to visit last month to clarify the "possible military dimensions" of Iran's nuclear programme, but were denied entry.
But on Monday, Iran's mission to the IAEA said if the UN agency "combined all related issues" then "once more, access would be granted".
Talks between the EU and Iran on the nuclear issue have been off and on for a number of years, with the last round ending in failure in January last year.
Baroness Ashton had written to Mr Jalili last October with an offer of new talks.
In February, Mr Jalili wrote back that Iran was ready for dialogue on what he called a spectrum of issues. He said he welcomed the P5+1's affirmation that it would respect Iran's right to the peaceful use of nuclear energy.
"No doubt that by committing to this approach, our talks for cooperation based on step-by-step principles and reciprocity on Iran's nuclear issues could be commenced," he wrote.
On Monday, Israeli PM Benjamin Netanyahu, on a visit to the US, said time was running out to put a stop to Iran's nuclear programme, warning Israel would "not live in the shadow of annihilation".
He stressed that all options were on the table, but that containment - leaving Iran to develop its programme under monitoring - was "not an option".
On Tuesday, US Defence Secretary Leon Panetta said: "Military action is the last alternative when all else fails. But make no mistake, we will act if we have to."