According to UN sources, in northern Syria, crimes and various forms of abuses including foreign fighters who call for what they call holy war against Syrian government has ratcheted up.

"Across northern Syria, there has been an upsurge in crimes and abuses committed by extremist anti-government armed groups along with an influx of rebel foreign fighters," Paulo Pinheiro, head of the inquiry, told the UN human rights Council.

"Entire brigades are now made up from fighters who have crossed into Syria, with Al Muhajireen being one of the most active," he said, as the international community is working round the clock to hamper a military strike on the Arab state through political negotiations.

The investigators have previously said foreign fighters from more than 10 countries; including Afghanistan and Russia's Chechnya region, as well as al-Qaeda-linked al Nusra forces are backing foreign-backed Syrian terrorist groups.

"Now it is probably more. The point is that these extreme elements have their own agenda and certainly not a democratic agenda that they are seeking to impose," commission member Vitit Muntarbhorn said. "That is a major worry from our side of the fence."

Pinheiro, reporting on suspected war crimes since July 15, also accused President Bashar al-Assad's government forces for shelling some towns during the war.

The UN report comes as a new study showed almost half of the rebel forces fighting against the Syrian government are hardline extremists with Al-Qaeda links.

According to analysis by defense consultancy IHS Jane's, the Syrian opposition consists of at least 10,000 fighters who have strong links to Al-Qaeda, while another 35,000 are hardline extremists, all split into around 1,000 fragmented cells.

MRKD/SHI