Syrian activists say clashes between government and rebel forces killed dozens of people Monday, underlining a new U.N. report that blamed both sides for increasing the number of attacks against civilians.

The Syrian Observatory for Human Rights reported that most of the clashes occurred in the north, but it said the reports of killings came from both the northern and southern parts of Syria.

While the claims by the British-based group cannot be independently verified, U.N. investigators did say they have documented increasing violence against civilians in the 18-month conflict between President Bashar al-Assad's forces and his opponents.

The Brazilian chairman leading a U.N. commission of inquiry, Paulo Pinheiro, told the U.N. Human Rights Council in Geneva that “egregious violations” happen so often that his team has not been able to investigate them all.

The panel's report, completed last month, said that while both government and anti-government forces committed war crimes, the abuses by opposition forces did not reach the “gravity, frequency and scale” of those carried out by pro-government sides.

The independent U.N. team also confirmed that an increasing number of “foreign elements,” including Islamist militants, are now operating in Syria — the first report to say that outsiders have joined the war.

The panel said some of these forces are joining armed anti-government groups, while others are operating on their own. Pinheiro said the presence of such jihadist militants tends to push rebels opposed to Mr. Assad toward more radical positions.

Rebels deny that foreigners had any role in starting the revolt, saying Syrians were seeking increased freedom from the regime. But as the conflict drags on, some rebels have acknowledged the presence of small numbers of foreigners among their ranks.

Syria's U.N. Ambassador Faysal Khabbaz Hamoui criticized the report, saying members of the international community are working to make the crisis in Syria worse.

The U.N. panel accused government forces and pro-regime militiamen of war crimes and crimes against humanity, including murder, summary executions, torture, arbitrary arrests, sexual violence and abuse of children. It also accused anti-government armed groups of war crimes including murder, extrajudicial execution and torture.

Separately, foreign ministers from Turkey and Iran met with Egyptian officials in Cairo to discuss the crisis. Saudi Arabia's Foreign Minister Prince Saud al-Faisal had planned to attend, but canceled because of health reasons. His deputy attended instead.

The United Nations says more than 20,000 people have been killed in the conflict; 1.2 million are uprooted within Syria, and more than 250,000 have fled abroad.