Annan said the international community must increase pressure on the Syrian government and rebel groups to stop the conflict.
"The specter of all-out civil war, with a worrying sectarian dimension, grows by the day," Annan told the Arab Ministerial Committee on Syria in Qatar. "The crisis is having regional spillover, in the form of tensions and incidents across the border, abductions of nationals and foreigners, and refugee flows to neighboring states."
Annan said he urged Syrian President Bashar Assad in Damascus this week to act immediately to implement the six-point plan he presented under the auspices of the United Nations and for Assad to honor his commitment to withdraw heavy weapons and cease all violence, a U.N. release said.
"The situation is complex, and it takes everyone involved in the crisis to act responsibly if the violence is to stop. But the first responsibility lies with the Syrian government, and with President Assad," Annan said Saturday. "He must release detainees, fully open up to international humanitarian assistance and allow people to express their views freely in peaceful protest. This is essential to demonstrate his seriousness to the Syrian people and the international community."
Annan said this week the crisis had reached a "tipping point" after a massacre in the Syrian village of Houla, which resulted in the deaths of 108 people, including 49 children.
The U.N. Human Rights Council has asked an independent U.N. panel probing abuses in Syria to investigate violations that may amount to crimes against humanity.
The United Nations estimates more than 9,000 people, mostly civilians, have been killed in Syria and tens of thousands displaced since the uprising against Assad began 15 months ago.
Notable deaths of 2014 [PHOTOS]