United Nations investigators said on Monday they had expanded a secret list of Syrians and military units suspected of committing war crimes during the 18-month-old conflict between President Bashar al-Assad's forces and his opponents.

The human rights investigators, led by Paulo Pinheiro, said they had gathered "a formidable and extraordinary body of evidence" and urged the U.N. Security Council to refer the situation in Syria to the International Criminal Court (ICC).

Britain and France said the time had come for Syria to be referred to the Hague-based U.N. war crimes court, but diplomats noted this would require acceptance by veto-wielding Russia and China, which have blocked all previous efforts to condemn Syria.

"Gross human rights violations have grown in number, in pace and in scale," Pinheiro told the U.N. Human Rights Council in Geneva. "There is no statute of limitations on these crimes."

He did not say if any Syrian rebels were among the names on the list, which updated a confidential one his independent team submitted to U.N. rights chief Navi Pillay in February.

Pinheiro presented the team's latest report, issued a month ago, saying Syrian government forces and allied militia have committed war crimes including murder and torture of civilians in what appears to be a state-directed policy.

More than 20,000 people have been killed in the 18-month-old conflict, 1.2 million are uprooted within Syria and more than 250,000 have fled abroad, the United Nations says.

Food, water and medical supplies have run short in areas subjected to Syrian government air strikes, shelling and siege, Pinheiro said, adding that investigators had received "numerous accounts...of civilians barely managing to survive".

Pinheiro reported an "increasing and alarming presence" of Islamist militants in Syria, some joining the rebels and others operating independently. They tended to radicalise the rebels, who have also committed war crimes, the Brazilian expert said.

He cited allegations that rebels had "used prisoners to detonate vehicle-borne explosives", thus killing their captives in acts that also posed a danger to civilians.

It would be "improper" to make public the list of suspects because they were entitled to the presumption of innocence and no mechanism to hold perpetrators responsible was yet in place where allegations could be contested, Pinheiro said.

His team interviewed more than 1,100 victims, refugees and defectors in the past year. "We have no interviews with wounded soldiers, or families of dead agents of the government because the government of Syria does not allow us access to Syria."

ENCOURAGING "JIHAD"

Syrian ambassador Faysal Khabbaz Hamoui accused Western and Arab powers of arming and funding rebels conducting a "jihad" or holy war against Damascus, and warned that this would backfire.

"The mercenaries are a time bomb that will explode later in the country and in countries supporting them after they finish their terrorist missions in Syria," he said.

"Terrorist groups" active in his country came from 17 countries, including Afghanistan, Libya and Yemen, he said.

The report should have named countries that "support the killers", which the Syrian envoy said included the United States, Qatar, Saudi Arabia, Turkey and Libya.

"One of the facts that 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, Qaeda elements, training them and funding them and sending them to Syria for jihad. This through fatwas that were issued," Khabbaz Hamoui said during the four-hour debate.

Russia, Syria's ally which has vetoed all Western attempts at the Security Council to condemn Syria, said rebels were committing "terrorist acts" including executions, and jihadists were increasingly active due to "support from the outside".

"There are jihadist mercenaries fighting on the opposition side. Those who in the view of some states are bringing democracy to the region are in actual fact carrying out mass murder," Russian diplomat Maria Khodynskaya-Golenishcheva said.

"They are deliberately firing on peaceful inhabitants who support the government...and are using hostages as suicide bombers and children as soldiers," she said.

Western nations want the session to condemn Assad's government and to extend the commission of inquiry's mandate, which expires this month.

European Union ambassador Mariangela Zappia said: "The international community must ensure impunity will not prevail."

U.S. human rights ambassador Eileen Chamberlain Donahoe also called for the investigators to pursue their work.

Turkey's ambassador Oguz Demiralp, describing the conflict in Syria as a "serious threat to international security", said those behind crimes there would be held accountable.