Last December, the U.N. Commission of Inquiry warned that the civil war had become "overtly sectarian."
It said government forces and militias, dominated by Alawites, had been attacking Sunnis -- who were "broadly (but not uniformly)" backing the rebel groups. And anti-government armed groups were targeting Alawites.
Other minority communities, including some Christians, Armenians, Palestinians, Kurds and Turkmen, "have also been caught up in the conflict, and in some cases forced to take up arms for their own defense or to take sides."
But it said the "sectarian lines fall most sharply" between Alawites and Sunnis.
The "increasingly sectarian nature" of the fighting is a motivator for proxy groups fighting in Syria. Anti-government armed groups are composed of Sunnis from the Middle East and North Africa, the report said.
Shiites from other countries have entered the conflict on behalf of Syria. The Lebanese Shiite group Hezbollah has said its members are fighting. There are reports of Iraqi Shiites fighting in Syria and of Iran's Revolutionary Guards providing intellectual and advisory support.
Christian communities across Syria have been under the gun and on the move. Homs, for example, was once home to 80,000 Christians, but the commission said most had escaped to Lebanon.
"With communities believing -- not without cause -- that they face an existential threat, the need for a negotiated settlement is more urgent than ever," the commission said.
Al-Assad makes public appearance
Also Saturday, the country's president made his second public appearance this month, according to state-run Syrian Television.
"President Bashar al-Assad joins thousands of students and families of martyrs in Damascus University in inaugurating the Martyred Students Monument in memory of all the students who were killed in Syria," it said.
The president's Facebook page posted a picture of the event.
On May 1, al-Assad visited the Ummayad electrical plant to "congratulate its staff and all the Syrian workers on the occasion of International Workers' Day," the state-run Syrian Arab News Agency reported.
Israel said to be flying over Lebanon
Israel was flying warplanes over Lebanon on Thursday, Friday and Saturday, the Lebanese army said.
Lebanon's president, Gen. Michel Sleiman, condemned the violations as "an attempt to shaken Lebanese stability," the state-run National News Agency reported Saturday.
The Israeli military had no comment. But a source in the Israeli defense establishment told CNN's Sara Sidner, "We will do whatever is necessary to stop the transfer of weapons from Syria to terrorist organizations. We have done it in the past and we will do it if necessary the future."
Israel appears to have struck Syria
Two U.S. officials told CNN on Friday that Israel appears to have conducted an airstrike into Syria on Thursday or Friday.
Based on initial indications, the U.S. does not believe Israeli warplanes entered Syrian airspace to conduct the strikes.