Western and Arab leaders ramped up the pressure on Syria and Iran at the United Nations, as US President Barack Obama vowed to keep Tehran from getting its hands on nuclear weapons.

The United States, France and Qatar led the charge Tuesday as the West and its allies attempted to use the UN General Assembly to win support for tougher international action against the Middle East's twin pariah regimes.

UN Secretary General Ban Ki-moon set the tone, condemning the bloodshed in Syria, where the beleaguered regime is battling an armed revolt and subjecting its citizens to what the UN peace envoy dubbed "medieval forms of torture."

Under pressure from his domestic rivals to take a tough stance, Obama said President Bashar al-Assad's regime "must come to an end so the suffering of the Syrian people can stop, and a new dawn can begin."

The emir of Qatar, Sheikh Hamad bin Khalifa Al-Thani, a key supporter of the Syrian opposition, called for Arab military intervention.

"It is better for Arab countries themselves to intervene out of their humanitarian, political and military duties and do what is necessary to stop the bloodshed," he told the General Assembly.

A US State Department official told AFP that the United States will soon announce an increase in its aid to the Syrian rebels, but would still stop short of sending weapons and ammunition.

And French President Francois Hollande urged the United Nations to declare protected areas in "liberated zones" under opposition control in Syria so that humanitarian aid could reach refugees.

Obama was also unequivocal on Iran, which is locked in a standoff with the West over a nuclear program that Washington alleges is designed to produce a weapon that could tip the balance of power in an already volatile region.

"A nuclear-armed Iran is not a challenge that can be contained. It would threaten the elimination of Israel, the security of Gulf nations, and the stability of the global economy," Obama warned.

"That is why a coalition of countries is holding the Iranian government accountable. And that is why the United States will do what we must to prevent Iran from obtaining a nuclear weapon," he declared.

Hollande also took a tough line on Iran, accusing it of supplying weapons and men to Assad to prop up his regime and dubbing this "unacceptable."

Six weeks ahead of the presidential election, Obama is under pressure on the foreign policy front, with criticism of his handling of the killing of US diplomats and claims he is not standing closely enough behind Israel.

His speech aimed to counter those claims from White House rival Mitt Romney and also renew his outreach to the Muslim world after two weeks of anti-American violence triggered by an online video that insulted Islam.

Obama said the Arab Spring would lead to improved democracy and living standards in a Middle East region more in line with US values but, while he condemned the film, he insisted no insults could justify violence.

He vowed that the militants who stormed the US consulate in Benghazi on September 11, killing the American ambassador to Libya and three colleagues, would face justice, and said the United States would always defend free speech.

"There are no words that excuse the killing of innocents. There is no video that justifies an attack on an embassy," he said.

Many Muslim leaders, however, demanded international action to stop religious insults, in a challenge to Obama's defense of freedom of expression.

President Susilo Bambang Yudhoyono of Indonesia, the world's most populous Muslim nation, quoted the Universal Declaration of Human Rights as saying that "everyone must observe morality and public order."

"Freedom of expression is therefore not absolute," he added.

Debate in New York in the run-up to the assembly focused on the violence in Syria and the risk that the Iranian stand-off could lead to a broader conflict if Israel or the United States launched a pre-emptive strike.

Ban spoke for many delegates when he called on world powers to put aside their differences and unite behind a plan to pressure the parties to settle their conflicts through negotiation.

The UN chief dubbed the Syria conflict "a regional calamity with global ramifications" and said: "The international community should not look the other way as violence spirals out of control."

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

The 15-nation Security Council has become paralyzed by deadlock over the 18-month-old war, which Syrian activists say has left more than 29,000 dead.

Ban also expressed concerns for the mounting tensions surrounding Iran, denouncing what he called: "The shrill war talk of recent weeks."

Iran's President Mahmoud Ahmadinejad, who is in New York, shrugged off talk of an attack on his country's nuclear facilities and said the Islamic republic would not end what it claims is peaceful civilian nuclear research.

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