Opposition groups in Syria have been accused of committing war crimes including torture and the summary execution of prisoners, and the UN has been warned of a growing number of human rights violations and the presence of foreign Islamist fighters ranged against the regime of Bashar al-Assad.

Human Rights Watch said it had documented more than a dozen executions by rebels in the northern provinces of Idlib and Aleppo and the coastal region of Latakia. Three opposition leaders who were confronted with evidence of extrajudicial killings said the victims had deserved to die, HRW reported.

The Free Syrian Army, the main armed opposition group, came under pressure from foreign supporters to end abuses after the public execution of 14 members of a clan of pro-regime militiamen in Aleppo in August. It then issued a code of conduct and pledged to respect human rights and humanitarian law.

"Declarations by opposition groups that they want to respect human rights are important, but the real test is how opposition forces behave," said Nadim Houry, deputy Middle East director at HRW. "Time and again Syria's opposition has told us it is fighting against the government because of its abhorrent human rights violations. Now is the time for the opposition to show that they really mean what they say."

Assad's government has been repeatedly condemned by human rights organisations and foreign governments for systematic abuses that are said to amount to war crimes and crimes against humanity.

In Geneva, meanwhile, the UN human rights council heard a report describing the "increasing and alarming presence" of "foreign Islamist elements" in Syria. That presence tended to radicalise homegrown rebels who had also committed crimes, said the investigator Paulo Pinheiro. "Gross violations of human rights have grown in number, in pace and in scale," Pinheiro told diplomats.

Pinheiro revealed a new secret list of Syrian individuals and units suspected of committing war crimes who should face criminal prosecution. It was based on "a formidable and extraordinary body of evidence", he said. He urged the UN security council to refer the situation in Syria to the international criminal court (ICC).

Against a background of bitter disagreements and effective paralysis at the UN, western countries are seeking further condemnation of Assad's government. Russia and China, which have used their security council vetoes to block any punitive action against Syria, seem certain to oppose any ICC referral.

The UN says 20,000 people have been killed in Syria in the past 18 months. Activists say the true figure is now closer to 30,000.

In Cairo, the foreign ministers of Egypt, Saudi Arabia, Iran and Turkey were meeting in the framework of a new diplomatic "contact group" proposed by Egypt to try to produce a regional solution to the Syrian crisis. Lakhdar Brahimi, the UN envoy who replaced Kofi Annan this month, was due to meet them to report on his meeting with Assad in Damascus on Saturday. Brahimi has described his mission as "nearly impossible".

Iran, Syria's closest regional ally, is said to be seeking to expand the group to include Iraq as well as Venezuela. The US, Britain and other western countries – and Syria's divided opposition – say Iran cannot help end the crisis since its support for Assad makes it part of the problem. Another glaring difficulty is that Saudi Arabia is a keen supporter of the anti-Assad opposition.