US President Barack Obama on Tuesday demanded "sanctions and consequences" for atrocities in Syria and said President Bashar al-Assad's rule must come to an end.
"The future must not belong to a dictator who massacres his people," Obama told the UN General Assembly in a keynote address.
"If there is a cause that cries out for protest in the world today, it is a regime that tortures children and shoots rockets at apartment buildings."
He told leaders at the UN headquarters: "As we meet here, we again declare that the regime of Bashar al-Assad must come to an end so that the suffering of the Syrian people can stop, and a new dawn can begin."
Obama also warned that the international community must act to prevent the 18 month old uprising against Assad turning into "a cycle of sectarian violence."
He said the United States wants 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."
"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," Obama said.
"We believe that the Syrians who embrace this vision will have the strength and legitimacy to lead."
Syria is one of the key topics at the UN assembly with growing western calls for action against Assad. Russia and China have vetoed three UN Security Council resolutions which could have led to sanctions.
On the other hand, Obama insisted there has been "progress" since the Arab Spring but said the recent turmoil in the Muslim world showed the hard task of achieving true democracy.
Under pressure over his handling of the Arab Spring from Republican opponents who have cast him as an apologist and an appeaser as they seek to oust him from office in November, Obama robustly defended his course.
Obama slammed the anti-Islamic film that has sparked unrest in Muslim countries as "disgusting" and vowed to hunt down those behind the attack in Benghazi that killed the US envoy to Libya and three other Americans.
But the president painted an optimistic overall picture of the current state of the Arab Spring, insisting that things were moving in the right direction despite the many challenges.
"So let us remember that this is a season of progress. For the first time in decades, Tunisians, Egyptians, and Libyans voted for new leaders in elections that were credible, competitive, and fair," he said.
His upbeat message was framed in caution and contained a plea to Muslim leaders to work with the United States and the West towards truer, fairer democracy.
"The events of the last two weeks speak to the need for all of us to address honestly the tensions between the West and an Arab World moving to democracy," he said.
"Just as we cannot solve every problem in the world, the United States has not, and will not, seek to dictate the outcome of democratic transitions abroad, and we do not expect other nations to agree with us on every issue.
"Nor do we assume that the violence of the past weeks, or the hateful speech by some individuals, represents the views of the overwhelming majority of Muslims -- any more than the views of the people who produced this video represent those of Americans," he said.
Obama warned that the turmoil of recent weeks showed how the path to democracy remained treacherous for many Arab Spring countries even after democratic elections.
"True democracy demands that citizens cannot be thrown in jail because of what they believe, and businesses can be opened without paying a bribe," he said.
"It depends on the freedom of citizens to speak their minds and assemble without fear; on the rule of law and due process that guarantees the rights of all people.
"In other words, true democracy -- real freedom -- is hard work. Those in power have to resist the temptation to crack down on dissent. In hard economic times, countries may be tempted to rally the people around perceived enemies, at home and abroad, rather than focusing on the painstaking work of reform."
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