UNITED NATIONS >> President Barack Obama opened the door to direct nuclear talks with Iran’s moderate new government today, declaring diplomacy worth pursuing though skepticism persisted about Tehran’s willingness to back up friendly overtures with concrete action.

“The roadblocks may prove to be too great, but I firmly believe the diplomatic path must be tested,” Obama said during an address to the United Nations General Assembly.

However, quiet negotiations over a possible first encounter between Obama and Iranian President Hasan Rouhani on the sidelines of the U.N. meeting ended without an agreement for the two leaders to meet. It would have marked the first time a U.S. and Iranian leader had met in 36 years.

Senior Obama administration officials said the U.S. and Iran had been discussing such an encounter for days and the White House supported the idea. But they said the Iranians informed the U.S. today that holding a meeting would be “too complicated.”

The U.S. officials insisted on anonymity because they were not authorized to discuss the deliberations publicly by name.

Rouhani, a moderate cleric elected in June, was making his international debut late today with his own address to the U.N. General Assembly. Since taking office, Rouhani has launched a charm offensive with the west, calling for a new start in relations with the U.S. and declaring that Iran is not seeking a nuclear weapon.

Rouhani’s overtures have been welcomed by the White House, stirring up speculation of a meeting with Obama. However, Rouhani skipped a U.N. leaders’ lunch this afternoon, erasing the most likely opportunity to meet with Obama.

The possible diplomatic thaw between the U.S. and Iran was being watched warily by Israel, which has long sought tough punishments against Tehran in retaliation for its nuclear program. Prime Minister Benjamin Netanyahu warned today that the world “should not be fooled” by signs of moderation by Rouhani.

“Iran thinks soothing words and token actions will enable it to continue on its path to the bomb,” Netanyahu said.

The U.S. and its allies have long suspected that Iran is trying to produce a nuclear weapon, though Tehran insists its nuclear activities are only for producing energy and for medical research.

Even without a meeting between Obama and Rouhani, it was clear that the U.S. and Iran were edging close to direct talks. Obama said he was tasking Secretary of State John Kerry with pursuing the prospect of a nuclear agreement with Iran. Kerry, along with representatives from five other world powers, is to meet Thursday with Iran’s new foreign minister, Mohammad Javad Zarif.

If Kerry and Zarif hold one-on-one talks on the sidelines of that meeting, it would mark the first direct engagement in six years between a U.S. secretary of state and an Iranian foreign minister.

A spokeswoman for Zarif said Thursday’s meeting indeed would mark the beginning of a “new era” in relations with the West.

Zarif was among the Iranian officials in the hall for Obama’s address today. A U.S. delegation will be in the hall for Rouhani’s speech, and the reaction will be closely watched. American officials sometimes walked out in protest during former Iranian President Mahmoud Ahmadinejad’s fiery anti-American speeches at the annual U.N. meetings.

Rouhani’s rhetoric has so far been more palatable to the U.S. But Obama warned today that it will take time to overcome the deep mistrust that has built up in the more than three decades since the U.S. and Iran broke off diplomatic relations.

“I don’t believe this difficult history can be overcome overnight,” Obama said. “The suspicion runs too deep. But I do believe that if we can resolve the issue of Iran’s nuclear program, that can serve as a major step down a long road toward a different relationship, one based on mutual interests and mutual respect.”

He added that in order for that effort to succeed, Iran’s “conciliatory words will have to be matched by actions that are transparent and verifiable.”

U.S. officials see Rouhani’s election and more moderate stance as a sign of frustration among the Iranian public over international isolation and crippling economic sanctions. However, the Obama administration is unclear whether Rouhani is willing to take the steps it is seeking in order to ease the sanctions, including curbing uranium enrichment and closing the underground Fordo nuclear facility.

The U.S. is also seeking indications that Rouhani, as he pursues better relations with the West, has the backing of Iran’s supreme leader, Ayatollah Ali Khamenei.

Obama has long said he is open to resolving the nuclear impasse with Iran through diplomatic channels, though he also has said in recent months that the window for that pathway is closing. Shortly after taking office in 2009, he exchanged letters with Khamenei, but their engagement quickly fizzled.

Obama again turned to letters this year to gauge Rouhani’s appetite for diplomacy. The Iranian leader responded to Obama’s outreach, setting the stage for their overlapping appearances at the United Nations.

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Associated Press writers Edith M. Lederer and Darlene Superville contributed.