U.S. President Barack Obama said Tuesday that he was encouraged by Iranian President Hassan Rohani, and directed Secretary of State John Kerry to pursue a diplomatic effort to resolve disputes over Iran's nuclear program.

In his address at the 68th UN General Assembly, the American president laid out U.S. policy toward the volatile Middle East and North Africa, and made clear that his administration will take direct action to eliminate threats when necessary and will use military force when diplomacy fails.

In closely watched remarks on Iran based on a diplomatic opening offered by Iran's new president, Obama said said the U.S. wants to resolve the Iran nuclear issue peacefully but is determined to prevent Iran from developing a nuclear weapon.
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"I do believe if we can resolve the issue of Iran's nuclear program that can serve as a major step in a long road toward a different relationship," Obama told the UN General Assembly.

He pointed to statements by Iran's leaders, but said in order to succeed, "conciliatory words will have to be matched by actions that are transparent and verifiable."

"The roadblocks may prove to be too great but I firmly believe the diplomatic path must be tested," Obama said.

Obama said he was asking Kerry to pursue diplomatic progress with Iran, in coordination with five other world powers. Kerry will join representatives from those nations Thursday in a meeting with Iranian Foreign Minister Mohammed Javad Zarif.

Zarif, who was in attendance during Obama's speech, tweeted a few hours before Obama's speech, "we have a historic opportunity to resolve the nuclear issue. 5+1 needs to adjust its posture commensurate with the new Iranian approach."

It's unclear whether Kerry and Zarif will meet one-on-one on the sidelines of Thursday's meeting.

On Syria, Obama urged the UN Security Council to approve a strong resolution aimed at ensuring the Assad regime keeps its commitments to give up chemical weapons and said America will provide an additional $340 million in humanitarian aid. Obama challenged the UN to include accountability in any resolution, saying the international body's reputation is at stake.

"If we cannot agree even on this," he said, "then it will show that the United Nations is incapable of enforcing the most basic of international laws."

The United States and Russia have brokered an agreement for Syria to give up its chemical weapons. But the countries are at odds on what the possible consequences would be if Syria doesn't comply.

"We believe that as a starting point the international community must enforce the ban in international weapons," he said in his address to the UN General Assembly.

The agreement between Washington and Moscow came as Obama was pushing Congress to approve a military strike against Syria for a chemical weapons attack on August 21 on civilians outside Damascus that the Obama administration says was carried out by Assad's regime. The subsequent diplomatic steps agreed to by U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov placed the threat of force on hold.

Still, the Russians have challenged the administration's claims of Assad culpability and Assad has blamed rebel forces for the attack. Obama aggressively pushed back against those claims in his speech Tuesday.

"It's an insult to human reason and to the legitimacy of this institution to suggest that anyone other than the regime carried out this attack," the president said.

Obama also said that while the international community has recognized the stakes involved in the more than 2-year-old civil war, "our response has not matched the scale of the challenge."

Obama reiterated his demand that Assad cannot continue to lead Syria, but said he would not use U.S. military force to depose him.

"That is for the Syrian people to decide," he said. "Nevertheless, a leader who slaughtered his citizens and gassed children to death cannot regain the legitimacy to lead a badly fractured country." He called on Assad allies to stop supporting his regime.

"The notion that Syria can somehow return to a pre-war status quo is a fantasy," he said. "It's time for Russia and Iran to realize that insisting on Assad's role will lead directly to the outcome that they fear: an increasingly violent space for extremists to operate."

Obama also called for the international community to get behind the "pursuit of peace" between Israel and the Palestinians, saying leaders of both sides are willing to "take significant political risks."

"Friends of Israel, including the United States, must recognize that Israel's security as a Jewish and democratic state depend on the realization of a Palestinian state," Obama said.

The U.S. president said that breakthroughs on both Israeli-Palestinian peace as well as on Iran's nuclear program would have "profound and positive impact" on the entire region.

Obama also vowed that the U.S. would continue to work with governments that do not meet, "at least in our view, the highest international expectations."

Along those lines, the U.S. would maintain a "constructive relationship" with Egypt's interim military government, but would not send military support until it was clear Egypt was pursuing a "more democratic path."

It is as yet unclear whether Obama would meet later in the day with Rohani, a moderate cleric who has been making friendly gestures toward the U.S. in recent weeks. Even a brief encounter would be significant given that the leaders of the U.S. and Iran haven't had face-to-face contact in more than 30 years.

U.S. officials say no meeting was planned, though they hadn't ruled out the possibility that one might be added. The most likely opportunity appeared to be at a UN leaders' lunch Tuesday.