Addressing world leaders at the 68th session of the UN General Assembly, Obama said if the international community cannot be united against forcing Syria to give up its chemical weapons, "then it will show that the United Nations is incapable of enforcing the most basic of international laws".
"The Syrian government took a first step by giving an accounting of its stockpiles. Now, there must be a strong Security Council resolution to verify that the Assad regime is keeping its commitments. And there must be consequences if they fail to do so," Obama said in his 50-minute address that focused on Iran, Syria and future US policies for the Middle East and North Africa.
He said the international community recognised the stakes in Syria early on, but "our response has not matched the scale of the challenge".
"With respect to Syria, (the US) believes that as a starting point the international community must enforce the ban on chemical weapons. When I stated my willingness to order a limited strike against the Assad regime in response to the brazen use of chemical weapons, I did not do so lightly," he said.
"I did so because I believe it is in the national security interests of the United States and in the interest of the world to meaningfully enforce a prohibition whose origins are older than the United Nations itself."
Obama said Syrian President Bashar Al Assad's "traditional allies" have "propped" him up, citing principles of sovereignty to shield his regime.
"The evidence is overwhelming that the Assad regime used such (chemical) weapons on August 21. UN inspectors gave a clear accounting that advanced rockets fired large quantities of sarin gas at civilians. These rockets were fired from a regime-controlled neighbourhood and landed in opposition neighbourhoods," he said.
"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. Now, I know that in the immediate aftermath of the attack there were those who questioned the legitimacy of even a limited strike in the absence of a clear mandate from the Security Council.
"But without a credible military threat, the Security Council had demonstrated no inclination to act at all," Obama said.
Obama backed off on plans for a military intervention in Syria after Russia proposed a plan to bring the Assad regime's chemical weapons under international watch. However he has warned that the US will act if Syria fails to destroy it chemical stockpile.
Obama said the US preference has always been a diplomatic resolution to the conflict in Syria.
"I do not believe that military action by those within Syria or by external powers can achieve a lasting peace. Nor do I believe that America or any nation should determine who will lead Syria. That is for the Syrian people to decide.
"Nevertheless, a leader who slaughtered his citizens and gassed children to death cannot regain the legitimacy to lead a badly fractured country. The notion that Syria can somehow return to a pre- war status quo is a fantasy. It's time for Russia and Iran to realise that insisting on Assad’s rule will lead directly to the outcome that they fear: An increasingly violent space for extremists to operate," he said.
Obama said as the international community seeks to pursue a settlement for the Syrian crisis, it is critical to note that "this is not a zero sum endeavour."
"We're no longer in a cold war. There's no great game to be won, nor does America have any interest in Syria beyond the well being of its people, the stability of its neighbours, the elimination of chemical weapons and insuring that it does not become a safe haven for terrorists."
Obama also announced an additional aid of $340 million to help Syria in its endeavours to pursue a peaceful path to democracy.
"No aid can take the place of a political resolution that give the Syrian people a chance to rebuild their country, but it can help desperate people to survive," he added.
Obama added that the United States is "chastised" for meddling in the Middle East region, and "accused of having a hand in all manner of conspiracy, at the same time the United States is blamed for failing to do enough to solve the region’s problems and for showing indifference towards suffering Muslim populations."
Outlining the US policy toward the Middle East and North Africa for the remainder of his presidency, Obama said America is "prepared to use all elements of our power, including military force, to secure our core interests in the region.
"We will confront external aggression against our allies and partners, as we did in the Gulf War."
"We will ensure the free flow of energy from the region to the world. Although America is steadily reducing our own dependence on imported oil, the world still depends on the region’s energy supply and a severe disruption could destabilise the entire global economy."
Obama stressed that America will "dismantle" terrorist networks that threaten its people.
"Wherever possible, we will build the capacity of our partners, respect the sovereignty of nations, and work to address the root causes of terror. But when it’s necessary, defend the United States against terrorist attack, we will take direct action," he added.
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