Always scrutinized, Iran now will be under even greater watch as the US looks for signals Iran’s new president is serious and powerful enough to pursue détente with Washington and an end to the painful economic penalties imposed over its nuclear program.

A burst of euphoria followed news of Friday’s telephone conversation between President Barack Obama and Iran’s Hassan Rowhani and the first top-level contact between the countries in 34 years led to talk of a historic breakthrough in relations.

But already the exuberance is being tamped down in Washington, where the dark cloud of skepticism over Iranian intentions won’t lift quickly or easily, and in Tehran, where there is no rush to say relations might be restored soon. Even the most upbeat inside the White House, while saying they have new hope for progress, insist that Rowhani must quickly reinforce his repeated declarations over the past week about being ready for compromise with actions proving his country is not seeking a nuclear weapon.

Iran has a chance to demonstrate its seriousness at the next round of nuclear talks with world powers, set for Oct. 15-16 in Geneva.

Confronting suspicions about his readiness to deal, Rowhani told a news conference before leaving New York on Friday that his government would present a plan in three weeks on how to resolve the impasse. Until then, a senior Obama administration official said, Obama’s foreign policy team will watch for signs that Rowhani truly is engaged in a “different calculation” — trying to decide whether defying the US and others on the nuclear issue is worth the pain of sanctions that have damaged Iran’s economy. The official spoke on condition of anonymity to discuss internal White House thinking.

The US, through the United Nations and unilaterally in some cases, has been the driving force behind the penalties that have isolated Iran from the world economy. “Now the big question is can the Iranians hold it together to make the very painful political concessions that will be necessary to win sanctions relief,” said R. Nicholas Burns, a Harvard professor and career US diplomat in both Democratic and Republican administrations who formerly served as lead negotiator on Iran's nuclear program. Rowhani insists that Iran’s supreme leader, Ayatollah Ali Khamenei, the gatekeeper for every decision on matters of state, has given his government the power to negotiate an end to more than three decades of US-Iranian estrangement.

But that cannot happen, Washington and its close allies say, as long as Tehran refuses to open its nuclear program to international inspections and proves its claim that it is enriching uranium only to fuel domestic energy and medical research programs. In Obama’s address to the General Assembly, he said no nation wanted to deny Iran a nuclear energy program. What remains at issue are deep suspicions that Iran is enriching uranium to levels far beyond what is needed to fuel electricity-generating reactors or to produce medical isotopes. Israel is particularly skeptical of Iran’s intentions, citing past declarations by Iranian officials that the Jewish state should be wiped from the map. Israel is constantly on guard against the Shiite militant group Hezbollah, which virtually rules southern Lebanon along Israel’s northern border.

The powerful Iranian Revolutionary Guard has armed Hezbollah with increasingly sophisticated rockets that could strike deep into Israel. The Revolutionary Guard could try to undermine Rowhani’s opening. So the rapidly shifting political ground raises questions about how the Guard might react to a policy that could challenge its regional footholds not only with Hezbollah in Lebanon but also its deep support of Syrian leader Bashar Assad in that country’s civil war.

• THE ASSOCIATED PRESS