(CNN) -- Western nations welcomed Friday a letter from Iran offering a resumption of stalled nuclear talks, though they were still determining the Islamic republic's sincerity.
European Union Foreign Policy chief Catherine Ashton said she was "cautious and optimistic" about the prospect of dialogue between Iran and six world powers -- the United States, France, Britain, China, Russia and Germany.
"Let me say that it's good to see that the letter has arrived and that there is the potential possibility that Iran may be ready to start talks," Ashton said at a news conference with U.S. Secretary of State Hillary Clinton.
Clinton called the letter from Iranian nuclear negotiator Saeed Jalili "an important step."
"This response from the Iranian government is one we've been waiting for and, if we do proceed, it will have to be a sustained effort that can produce results," she said.
Earlier, the United Nations chief had said the prickly issue of Iran's nuclear ambitions must be resolved through diplomacy.
Ahead of a critical visit to Tehran by inspectors from the International Atomic Energy Agency (IAEA), Secretary-General Ban Ki-moon said the onus was on Iran to disprove military dimensions of its nuclear program.
"To my mind and to the IAEA, they have not been able to convince the international community, so they have to fully cooperate with the IAEA and with the United Nations and the Security Council," Ban told reporters after ceremonies marking the 15th anniversary of the Comprehensive Test Ban Treaty Organization in Vienna, Austria.
"That is their responsibility," he said about Iran.
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Jalili's letter opens up prospects for talks at a time when tensions have escalated greatly between Iran and its foes.
"We voice our readiness for dialogue on a spectrum of various issues, which can provide ground for constructive and forward-looking cooperation," Jalili wrote to Ashton in response to an invitation she sent in October.
That letter followed Iranian announcements this week of nuclear progress that included the development of sophisticated centrifuges and fuel rod production.
President Mahmoud Ahmadinejad was on hand Wednesday to load the first domestically-made fuel rods into the core of a Tehran reactor.
Iran also announced its intent to start production of yellowcake, a chemically treated form of uranium ore used for making enriched uranium.
U.N. sanctions ban Iran from importing yellowcake. Domestic production would further Iran's nuclear self-sufficiency.
Iran hailed the developments as significant, but the U.S. State Department dismissed them as bluster for a domestic audience.
"We frankly don't see a lot new here," said spokeswoman Victoria Nuland. "The Iranians for many months have been putting out calendars of accomplishments and based on their own calendars they are many, many months behind."
Still, the United States and its allies claim Iran is developing a nuclear weapon; Iran has insisted all along that its nuclear program is solely intended for civilian energy purposes.
A scathing IAEA report in November that lent credence to Western beliefs about Iran's intentions escalated the nuclear crisis.
The situation grew even more grave as speculation arose that Israel may launch a preemptive strike to set back Iran's nuclear program.
Western punitive measures have stung the Iranian economy, though it was unclear whether Iran's offer for talks were a result of sanctions or a play for time.
Swedish Foreign Minister Carl Bildt said in Vienna Friday that sanctions can "never be a policy in itself."
"We have pursued from the EU side a twin-track policy: sanctions in order to reinforce the diplomatic search for solutions," Bildt said. "And whichever way you look at it there is no other solution but a diplomatic and a political one."
Anthony Cordesman, a senior analyst at the Center for Strategic and International Studies, said Western nations should allow enough time for the sanctions to have effect.
"I think the best option is the fact for the first time we have sanctions which potentially cripple Iran's economy," Cordesman said. "These won't go into full action and full effectiveness for six months. We need to give them time."
CNN's Elise Labott contributed to this report.
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