Secretary-General Ban Ki-moon demanded international action to stop the war in Syria, telling a somber gathering of world leaders Tuesday that the 18-month conflict had become "a regional calamity with global ramifications."

In sharp contrast to the U.N. chief, President Barack Obama pledged U.S. support for Syrians trying to oust President Bashar Assad -- "a dictator who massacres his own people."

Opening the U.N. General Assembly's annual ministerial meeting, Ban said in his state of the world speech that he was sounding the alarm about widespread insecurity and injustice, inequality and intolerance in many countries.

Putting the spotlight on Syria, the U.N. chief said "the international community should not look the other way as violence spirals out of control."

"We must stop the violence and flows of arms to both sides, and set in motion a Syrian-led transition as soon as possible," he said.

While Obama didn't call for an end to the violence, he made no mention of arming the opposition and stressed the importance of ensuring "that what began with citizens demanding their rights does not end in a cycle of sectarian violence."

"Together, we must stand with those Syrians who believe in a different vision -- a Syria that is united and inclusive, where children don't need to fear their own government, and all Syrians have a say in how they are governed, Sunnis and Alawites, Kurds and Christians," said Obama, who arrived at the U.N. after Ban spoke.

"That is what America stands for; that is the outcome that we will work for -- with sanctions and consequences for those who persecute; and assistance and support for those who work for this common good," the U.S. president said.

Declaring that the situation in Syria is getting worse every day, the secretary-general called the conflict a serious and growing threat to international peace and security that requires attention from the deeply divided U.N. Security Council.

That appears highly unlikely, however, at least in the near future.

Russia and China have vetoed three Western-backed resolutions aimed at pressuring Syrian President Bashar Assad to end the violence and enter negotiations on a political transition, leaving the U.N.'s most powerful body paralyzed in what some diplomats say is the worst crisis since the U.S.-Soviet standoff during the Cold War.

Brazil's President Dilma Rousseff, whose country by tradition is the first to speak, supported the secretary-general saying: "There is no military solution to the Syrian crisis. Diplomacy and dialogue are not just our best option: they are the only option."

Ban also expressed profound concern at continuing violence in Afghanistan and Congo, increasing unrest across west Africa's Sahel region where al-Qaida has made inroads, and the "dangerous impasse" between Israelis and Palestinians that may close the door on the two-state solution.

The "shrill war talk" by Israel in recent weeks, in response to its belief that Iran is pursuing nuclear weapons, "has been alarming," Ban said, and Tehran's rhetoric threatening Israel's existence is unacceptable.

"Any such attacks would be devastating," he said, reminding the presidents, prime ministers, monarchs and diplomats from the 193 U.N. member states of the need for peaceful solutions and respect for international law.

"Leaders have a responsibility to use their voices to lower tensions instead of raising the temperature and volatility of the moment," he said.

Alluding to the recently circulated amateur video made in the U.S. which attacks Islam and denigrates the Prophet Muhammad, Ban said that "in recent days we have seen hate speech and violent responses that perpetuate a cycle of blind violence."

He lamented that in the world today "too often, divisions are exploited for short-term political gain" and "too many people are ready to take small flames of difference and turn them into a bonfire."

The secretary-general said it's time for responsible political and community leaders and ordinary citizens to speak out.

"The moderate majority should not be a silent majority," Ban said. "It must empower itself, and say to bigots and extremists alike: `you do not speak for us."'

Obama urged all leaders "to speak out forcefully against violence and extremism" and join the U.S. in confronting the root causes of the rage across the Muslim world.

He condemned the anti-Muslim video that helped spark the recent attacks, calling it "cruel and disgusting." But he strongly defended the U.S. Constitution's protection of the freedom of expression, "even views that we profoundly disagree with."

Obama was not expected to cross paths with Iranian President Mahmoud Ahmadinejad, who addresses the assembly on Wednesday morning, but he did have a message about the country's nuclear program: There is still "time and space" to resolve the dispute over Iran's nuclear ambitions "but that time is not unlimited."

Ahmadinejad insists his country's nuclear program is only for electricity generation and medical research, but the U.S. and Western allies are demanding that Iran open all its facilities to inspectors from the U.N. nuclear agency to prove the intent of its enrichment of uranium.

Obama said a nuclear-armed Iran "would threaten the elimination of Israel, the security of Gulf nations, and the stability of the global economy" and would also risk triggering a nuclear arms race in the region.

"And that is why the United States will do what we must to prevent Iran from obtaining a nuclear weapon," he said.

Brazil's Roussef hit another topic certain to be in the forefront of speeches by many world leaders -- the slow recovery from the global economic crisis.

"There will be no effective response to the economic crisis without strengthened coordination efforts between United Nations members and multilateral bodies such as the G20, the IMF and the World Bank," Roussef warned. "It is urgent that we build a comprehensive pact for the coordinated resumption of global economic growth, in order to forestall the despair caused by unemployment and the lack of opportunities."

On a positive note, secretary-general Ban said he took heart from the important steps forward on several fronts including the democratic transitions that are underway in the Arab world, Myanmar and many other countries, a fledgling government in Somalia, Africa's economic growth becoming the fastest in the world and Asia and Latin America "making important advances."

But the U.N. chief told the leaders "we need more from each and every one of you. And the world needs more from our United Nations."