Speaking to the United Nations General Assembly, President Obama sounded a cautiously optimistic tone about the prospects for diplomacy, saying he had instructed Secretary of State John Kerry to pursue face-to-face negotiations with Iran on its nuclear program.

“The roadblocks may prove to be too great,” he added, “but I firmly believe the diplomatic path must be tested.”

President Obama also called on the Security Council to pass a “strong” resolution that would impose consequences on Syria if it fails to turn over its chemical weapons. The American threat of military action against Syria, Obama said, had set in motion diplomatic efforts with Russia to take over and eventually destroy Syrian President Bashar al-Assad’s weapons.

“Without a credible military threat, the Security Council had demonstrated no inclination to act at all,” the president said. “If we cannot agree even on this, then it will show that the UN is incapable of enforcing the most basic of international laws. On the other hand, if we succeed, it will send a powerful message that the use of chemical weapons has no place in the 21st century, and that this body means what it says.”

Barack Obama also announced that the United States would pledge an additional $US340 million ($372 million) in humanitarian aid to help refugees from the civil war in Syria. And while he praised the diplomatic initiative by Russia on chemical weapons, he also said that the continuing support of Russia and Iran for the government of President Bashar al-Assad risked leading to further extremism there.

Obama’s speech came at a time of swift, almost disorienting diplomatic developments, with the White House first threatening, then backing off a military strike against Syria, and suddenly encountering a diplomatic opening with Iran on its nuclear program. The US president tried to take account of all of it, in a wide-ranging speech that echoed some of the themes of his address last spring on the changing American role in the world.

“For the United States,” he said, “these new circumstances have also meant shifting away from a perpetual war-footing.”

Part of the American recalculation, he said, involved restricting the use of drones in counter-terrorism operations; transferring prisoners from the military prison at Guantánamo Bay, Cuba and ultimately shutting it down; and rethinking its broad surveillance activities.

“Just as we reviewed how we deploy our extraordinary military capabilities in a way that lives up to our ideals,” the president said, “we have begun to review the way that we gather intelligence, so as to properly balance the legitimate security concerns of our citizens and allies, with the privacy concerns that all people share.”

But Obama emphasised three areas: the civil war and the use of chemical weapons in Syria, the prospect of diplomacy with Iran, and the Middle East peace process between Israelis and Palestinians, which has recently restarted under the prodding of Mr. Kerry.

“The time is now ripe for the entire international community to get behind the pursuit of peace,” he said. “Already, Israeli and Palestinian leaders have demonstrated a willingness to take significant political risks.”

But much of Obama’s focus was on when the United States would use its power in the Middle East. Acknowledging that his reversal on Syria had prompted uneasiness in the region about American resolve, Obama insisted that the United States would still act to protect its interests and, in some cases, to avert humanitarian tragedies.

“The United States of America is prepared to use all elements of our power, including military force, to secure these core interests in the region,” he said. “We will confront external aggression against our allies and partners, as we did in the Gulf War.”

Speaking immediately after Obama, Turkey’s president, Abdullah Gul, welcomed the Russian-American agreement to disarm Syria’s chemical weapons stockpile.

But Gul, whose government has emerged as one of Assad’s biggest opponents, also said the Syrian conflict had morphed into a “real threat to regional peace and security” and that the United Nations had a responsibility to help resolve the crisis, which has left more than 100,000 people dead and millions displaced.

On Iran, Obama mixed hope with wariness, saying that three decades of estrangement would not be repaired quickly.

“I don’t believe this difficult history can be overcome overnight – the suspicion runs too deep,” Mr. Obama declared. “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.”

The president spoke immediately after Brazil’s president, Dilma Rousseff, who delivered a denunciation of the United States over reports that the National Security Agency monitored emails, text messages and other electronic communications between Ms Rousseff and her aides. Last week, President Rousseff cancelled a state visit to Washington to signal her displeasure with the NSA surveillance.

President Obama also sent a warning to Egypt’s military-backed government that it would lose American support if it continued to crack down on dissident elements there.

“We will continue support in areas like education that benefit the Egyptian people,” he said. “But we have not proceeded with the delivery of certain military systems, and our support will depend upon Egypt’s progress in pursuing a democratic path.”

As Obama spoke, the United Nations was crackling with speculation that he might shake hands with President Hassan Rouhani of Iran, who was scheduled to address the General Assembly later on Tuesday. There is no formal meeting with Rouhani on his schedule, but White House officials have said they are open to engagement “at a variety of levels”.

Rouhani was not in the General Assembly hall for Obama’s speech, though Iran’s foreign minister, Mohammad Javad Zarif, was.

The last time an American president met an Iranian leader was on December 31, 1977, when Jimmy Carter and wife Rosalyn spent New Year’s Eve as a guest of the Shah of Iran. As he made a toast to the shah at a state dinner, President Carter said the idea for the trip came from Mrs Carter, when he asked her with whom she would like to celebrate the holiday.

“We have no other nation on Earth who is closer to us in planning for our mutual military security. We have no other nation with whom we have closer consultation on regional problems that concern us both,” Carter said.” And there is no leader with whom I have a deeper sense of personal gratitude and personal friendship.”

Less than two years later, an angry crowd overran the American embassy in Tehran, holding 52 Americans hostage for 444 days and plunging relations between Iran and the United States into a deep freeze from which they have not yet emerged.