"This year, we have the best opportunity in decades to pursue a different future between our countries," he said in a message to mark the Persian new year festival of Nowruz.
Speaking directly to the Iranian people and their leaders, Obama said the days and weeks ahead will be critical.
"Our negotiations have made progress, but gaps remain. And there are people, in both our countries and beyond, who oppose a diplomatic resolution," he said, without referring to any country or individual.
"My message to you — the people of Iran — is that, together, we have to speak up for the future we seek," he said hours after Secretary of State John Kerry and his Iranian counterpart Javad Zarif hailed "progress" in the talks.
"As you gather around the Nowruz table — from Tehran to Shiraz to Tabriz, from the coasts of the Caspian Sea to the Persian Gulf — you’re giving thanks for your blessings and looking ahead to the future," Obama said.
The US and Iran have no diplomatic relations since 1979 Iranian revolution and seizure of US embassy in Tehran.
"Just over a year ago, we reached an initial understanding regarding Iran's nuclear program. And both sides have kept our commitments. Iran has halted progress on its nuclear program and even rolled it back in some areas," he said.
"The international community, including the US, has provided Iran with some relief from sanctions. Now, our diplomats — and our scientists — are engaged in negotiations in the hopes of finding a comprehensive solution that resolves the world’s concerns with Iran’s nuclear programme," he said.
Obama said he believe that the two countries should be able to resolve this issue peacefully, with diplomacy.
"Iran's Supreme Leader Ayatollah Khamenei has issued a fatwa against the development of nuclear weapons, and President (Hassan) Rouhani has said that Iran would never develop a nuclear weapon," Obama recalled.
"Together with the international community, the US has said that Iran should have access to peaceful nuclear energy, consistent with Iran’s international obligations. So there is a way for Iran — if it is willing to take meaningful, verifiable steps — to assure the world that its nuclear program is, in fact, for peaceful purposes only," he added.
The P5+1 - China, Russia, the UK, the US, France and Germany - are trying to broker a deal with Iran to end standoff over its nuclear programme. Iran maintains that its nuclear programme is for peaceful purposes.

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