The UN secretary-general has warned that Syria's people feel increasingly abandoned by the world as their country's crisis enters its fifth year.
Ban Ki-moon called on President Bashar al-Assad by name to take decisive steps to end the conflict.
"Governments or movements that aspire to legitimacy do not massacre their own people," Ban declared on Thursday.
In a statement, Ban described the international community as "divided and incapable of taking collective action" in the civil war.
That division is seen in the UN Security Council, which has been largely powerless to take strong action because of the threat of a veto from permanent member Russia, Syria's ally.
Ban's spokesman, Stephane Dujarric, told reporters that the UN chief was not so much assigning blame but underscoring "our collective responsibility”.
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The Syrian conflict began months after popular protests erupted in March 2011.
More than 220,000 people have been killed so far in the war, and more than 4 million have fled the country.
The UN says it needs another $2.9bn to help Syrians caught up in the conflict.
Government forces and rebels are battling each other on many fronts, and the Islamic State of Iraq and the Levant (ISIL) group has seized large swathes of territory.
Ban warned of the "fearsome prospect of the total collapse of this country" and its effects throughout the region.
He said the lack of accountability in the conflict has led to an exponential increase in war crimes and crimes against humanity.
"Each day brings reports of fresh horrors," he said, including executions, systemic torture, the use of indiscriminate weapons like barrel bombs on civilians, siege and starvation and the use of chemical weapons.
The UN chief's statement came as more than 20 international aid groups issued a joint condemnation of the Security Council for its failure to back up the resolutions it passed last year to help get aid to millions of Syrians and protect civilians from the fighting.
The aid groups, including the International Rescue Committee, the Norwegian Refugee Council and Handicap International, called on UN members to ensure the resolutions are fully implemented.