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The head of a U.N.-appointed panel probing alleged war crimes in Syria warned Tuesday that the presence of hundreds of radical Islamists threatens to radicalize the 19-month conflict which is already witnessing a dramatic rise in ethnic tensions.

Brazilian professor Paulo Sergio Pinheiro said at a news conference that the panel is afraid these foreign militants and radical Jihadists are not fighting for democracy and freedom but have their own radical agendas.

"I think this is a presence that worries us very much," he said. "Their presence can contribute to the radicalization in this conflict ... (and) is particularly dangerous in an already very volatile conflict."

From the 1,100 interviews the panel has conducted, Pinheiro said, the panel found that foreign combatants come from 11 countries but it doesn't have specific numbers.

Pinheiro said human rights violations by both sides "have escalated dramatically" in recent months though the scale and gravity of the opposition's violations is less than the government's.

Pinheiro and fellow commission member Karen AbuZayd said they had seen no sign that codes of conduct adopted by the Free Syrian Army and the government were being implemented.

"No one seems to be exercising any control over some of the worst things that are happening," Abu Zayd said.

The Independent Commission of Inquiry on Syria, appointed by the U.N. Human Rights Council, recently had its mandate to document, register and monitor alleged rights violations and war crimes extended until March 2013.

Pinheiro stressed that the panel is not a war crimes tribunal that can prosecute alleged perpetrators, though it has produced two lists of individuals and units believed responsible for serious violations.

The only body that can prosecute offenders is the International Criminal Court, but that requires a referral from the deeply divided U.N. Security Council where Russia and China have vetoed three resolutions trying to pressure Syrian President Bashar Assad to end the fighting that activist say has killed more than 30,000 people.

Pinheiro said that at a private meeting with council members last Friday he and fellow commission member Karen AbuZayd "delicately mentioned" that it was their responsibility to ensure accountability for alleged war crimes and violations of international human rights law.

The commission conducted the 1,100 interviews outside the country, including a few by phone and by Skype to people inside, he said.

AbuZayd said the number of refugees has increased dramatically from 100,000 in June to 350,000 today, adding that Syrians are fleeing not only because of the fighting but because of the shortage of food, fuel, water, an economy going downhill, and a 30 percent decline in the value of Syria's currency, the pound. She added that 589,000 buildings, large and small, are documented to have been destroyed.