Rebels in Syria are becoming increasingly radicalised as more and more foreign fighters join the battle to overthrow President Bashar al-Assad's regime, United Nations human rights investigators have warned.

The UN team, expressing concern at the "increasing and alarming presence" of Islamist militants in Syria, said the numbers of human rights abuses committed by both sides had risen dramatically in recent weeks.

The independent panel of experts, commissioned by the UN Human Rights Council, said it had drawn up a secret new list of Syrians and military units suspected of committing war crimes, and called for the UN Security Council to refer the country to the International Criminal Court (ICC).

"Gross violations of human rights have grown in number, in pace and in scale," Paulo Sergio Pinheiro, the Brazilian diplomat leading the investigators, told the Human Rights Council in Geneva. "Civilians, many of them children, are bearing the brunt of the spiralling violence." His stark warning comes at a time of heightened violence in Syria's 18-month-long civil war, as the international community remains divided about how, if at all, to intervene. Pro-opposition activist groups say more than 5,000 people were killed in August – the highest monthly total yet. The same groups claim that about 27,000 people have been killed since the start of the conflict, while the UN puts the number at closer to 20,000.

While presenting the report, Mr Pinheiro said the situation in Syria had deteriorated significantly and many groups of foreign fighters were operating independently. "Such elements tend to push anti-government fighters toward more radical positions," he said.

He noted that the anti-government Free Syrian Army appeared to have adopted a code of ethics, but that groups affiliated to it were reported to have executed 21 captured government soldiers in Aleppo earlier this month. Other groups forced prisoners to drive vehicles packed with explosives towards government checkpoints before detonating them. But his strongest criticism was reserved for the Syrian government, which he said was responsible for indiscriminate air strikes and assaults by ground forces that endangered civilians. "It is apparent that the crimes and abuses committed by anti-government groups, though serious, did not reach the gravity, frequency and scale of those committed by the government forces and Shabbiha [militia]," Mr Pinheiro said.

Responding to the report's findings, Syria's ambassador to the UN in Geneva, Faysal Khabbaz Hamoui, hit out at Western and Arab countries, accusing them of prolonging the conflict through financial and material support for the rebels.

"One of the facts we do not see in the report is that many international parties are working at increasing the crisis in Syria through instigating their media, through training mercenaries, al-Qa'da elements, training them and funding them and sending them to Syria for jihad," he said.

Away from the UN Human Rights Council, a separate report released yesterday by the New York-based Human Rights Watch heaped further accusations of abuses on armed opposition groups in Syria.

The report accused the groups of subjecting detainees to ill-treatment, torture and summary executions in the northern areas of Aleppo, Idlib and Latakia. It said it had conducted interviews with several detainees held by the Free Syrian Army (FSA) who claimed to have been tortured.

The FSA condemned the crimes documented in the report, blaming foreign militants who claimed to be part of the FSA but who operated independently of it. "All our groups have clear orders not to do anything that violates international law," said Louay Almokdad, a spokesman for the FSA's military council. "The people carrying out these crimes are out of control and do not belong to the Free Syrian Army. Anyone who commits these crimes should face judgment, and we will hand them over if we catch them.