US President Barack Obama has denounced world powers that continue to support embattled Syrian leader Bashar al-Assad.
Syria "cannot return to the pre-war status quo", Obama said at the 70th United Nations General Assembly on Monday afternoon.
The US leader decried Assad as "a tyrant" and "a brutal dictator [who] slaughters tens of thousands of his own people", adding that peace in Syria is only possible if Assad leaves office.
The Syrian conflict, which began with peaceful protests in 2011, has led to at least 250,000 deaths, according to the UN. More than half of Syria's pre-war population of 22.4 million has been internally displaced or fled abroad.
"Assad and his allies cannot simply pacify the broad majority of a population who has been brutalised," Obama said. He called for a "managed transition" that removes Assad from the presidency and puts in his place a more democratic system.
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The US president's comments underscore the tensions between the US and Russia, the latter of which has stepped up its support for the Assad government with an ongoing military build-up in Syria.
Russian President Vladimir Putin mounted a fierce defence of Assad.
The Russian leader urged UN General Assembly members to unite to fight the Islamic State of Iraq and the Levant (ISIL) group and said it would be a disastrous error not to support Syria's beleaguered president.
Iran piled on behind Russia, but Western leaders remained adamant Assad must step down and Obama branded him a child-killing tyrant whose actions had fuelled ISIL's rise.
Nonetheless, Obama also said that the US will work with any nation on Syria, including Russia and Iran.
He also denounced ISIL: "I said it before, and I will say it again: There is no room for accommodating an apocalyptic death cult like ISIL.
"We know that ISIL, which emerged out of chaos in Iraq and Syria, depends on perpetual war to survive," he said.
Obama said that the US makes "no apologies" for its military action in Syria, including its ongoing air strikes against ISIL and the $500m Pentagon programme to train and equip around 5,000 fighters to battle ISIL.
The US programme has been sharply criticised as less than 150 fighters have completed it throughout the last year.
Last week, a US-trained rebel unit surrendered six pick-up trucks and ammunition to al-Nusra Front. Back in July, a 54-person group of US-trained fighters who entered Syria to fight ISIL were confronted by Nusra. Several were killed and others were captured.
"We have demonstrated over more than a decade of relentless pursuit of al-Qaeda [that] we will not be outlasted by extremists," Obama said.
Source: Al Jazeera and agencies