The head of the UN atomic watchdog, Yukiya Amano, also said yesterday (17 September) that his agency would hold more talks with the Islamic state aimed at allaying Western concerns that Iran is attempting to develop nuclear weapons, a charge Tehran denies.
Ashton will meet Iran negotiator Saeed Jalili in Istanbul as "part of continuing efforts to engage with Iran," after talks between world powers and Iran in Moscow in June failed to secure a breakthrough in the dispute.
US Secretary of State Hillary Clinton has called Iran's proposals so far "nonstarters."
US State Department spokeswoman Victoria Nuland said the purpose of the Ashton-Jalili meeting was to determine whether the Iranians were willing to offer any new proposals.
"That's the question that we have. We made clear that what they had put on the table was a nonstarter. Are they prepared to bring anything new?" Nuland told reporters in Washington.
She added that the major powers - the five permanent members of the UN Security Council plus Germany - were expected to meet in New York later this month on the sidelines of the UN General Assembly to discuss Iran.
As diplomatic efforts to solve the dispute have stalled, Israeli Prime Minister Benjamin Netanyahu has gone public with a demand that the United States set "red lines" that Iran not cross or risk military action. The United States has refused to set such triggers, though both it and Israel have said they reserve the right to take military action if necessary.
Merkel told a news conference in Berlin that Iran posed a threat not just to Israel but to the whole world, but added: "I support a political solution ... and I believe that we are not at the point where the search for political solutions has been exhausted."
International Atomic Energy Agency chief Amano said the UN agency, which is investigating Tehran's atomic activities, was "firmly committed" to intensifying dialogue with the Islamic state despite the lack of progress.
Amano, addressing his organisation's annual member-state gathering in Vienna, gave no date for a possible new talks between IAEA officials and Iran, which says its nuclear programme is for peaceful purposes. The last meeting was in August.
Iran's relations with the IAEA have become increasingly strained in recent years as the agency has voiced growing concern about possible military dimensions to Tehran's nuclear programme.
The European Union reiterated its demand in a statement at the gathering that Iran must suspend uranium enrichment - something the Islamic state has repeatedly rejected. That demand is in a series of UN Security Council resolutions approved since 2006.