UN Secretary General Ban Ki-moon said the Syrian regime "continues to barrel bomb neighbourhoods and systematically torture thousands of detainees."
United Nations Secretary-General Ban Ki-moon accused the Syrian regime on Tuesday of killing the most civilians during the country's five year conflict.
"Powerful patrons that keep feeding the war machine also have blood on their hands."
In his final address to the annual gathering of world leaders at the United Nations, Ban said the Syrian regime "continues to barrel bomb neighbourhoods and systematically torture thousands of detainees."
Meanwhile, Turkish President Recep Tayyip Erdoğan has reiterated his calls for Bashar al Assad to be removed from power to achieve peace in Syria.
Speaking in an exclusive interview, Erdoğan told Reuters that the future of Syria should be "determined by its own people."
"Why this killer is being backed by some states?" the Turkish president asked, referring to international support being given to Assad by states such as Russia, China and Iran.
"Assad cannot be part of any transitional period ... the world should find a solution that does not involve Assad ... Syria's territorial integrity should be respected by other countries."
Over five years since the Syrian civil war started in the context of the "Arab Spring" revolution, Erdoğan's stance on the Assad regime has remained unchanged.
Over 400,000 people have been killed in the war, which has also displaced half the country's population, according to monitoring groups. With almost three million Syrian refugees, Turkey has bore much of the brunt of the war.
The country has also faced a significant increase in the threat to its national security, with hostile groups such as DAESH and the PKK-affiliated YPG taking control of Turkey's southern border with Syria.
Last month, the Turkish Army launched Operation Euphrates Shield in cooperation with Free Syrian Army (FSA) opposition forces to oust the DAESH terrorist group from a stretch of land along its border.
As the Turkish-backed FSA fighters advance further south towards the northern Aleppo town of Al Bab, they risk clashing with the US-backed YPG, which has failed to heed Turkish warnings to retreat east of the Euphrates River.
Despite being linked to the PKK, which is listed as a terrorist organisation by Turkey, the EU and the US, the YPG has been treated by the US as an ally in the battle against DAESH. The difference in policy has put Ankara and Washington, both NATO allies, at odds with each other.
"DAESH and YPG ... are the main source of threat," the Turkish president said.
"We have been patient... We have not deployed all of our troops to Syria ... with the moderate opposition Jarablus was freed," he told Reuters, adding that Turkey is willing to participate in a US-led coalition to liberate the Syrian city of Raqqa from DAESH.