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Navi Pillay's representative: "Those who order, help or fail to stop attacks on civilians are individually criminally liable for their actions"
Last week's killing of more than 100 civilians at Houla in Syria may amount to crimes against humanity, the UN high commissioner for human rights has said.
Navi Pillay urged the international community to "make all efforts to end impunity" and "ensure accountability for perpetrators" of such "atrocities".
Ms Pillay was addressing an emergency session of the UN Human Rights Council.
Earlier, opposition activists said there had been another mass killing of civilians by pro-government militiamen.
Thirteen factory workers were forced off a bus and executed by "shabiha" - the name given to these armed regime supporters - on Thursday in al-Buwaida al-Sharqiya, near the western town of Qusair, they added.
Several videos posted online showed bodies with severe wounds to the head and stomach, consistent with being shot at close range.
The activists' account cannot be independently verified, but twice in the past week UN observers on the ground have corroborated similar claims.
Individuals 'criminally liable'On Friday, the UN Human Rights Council, the world's top human rights body, began meeting in emergency session to discuss the violence Syria - the fourth time it has done so since the uprising against President Bashar al-Assad began in March 2011.
The 47-member council is expected to condemn in the strongest possible terms the massacre in the Houla area of Homs province.
Residents of the village of Taldou said shabiha had been sent in early on Saturday after the Syrian army unleashed a barrage of heavy weapons late on Friday in response to a local anti-government protest.
Navi Pillay's office reported on Tuesday that UN investigators had found most of the 108 victims had been shot at close range or stabbed. No more than 20 had been killed by the tank and artillery fire which preceded the raid, it added.
In a speech to the Human Rights Council read out by a representative, Ms Pillay said she was "appalled by the atrocities committed in Houla".
Taldou, Houla region
- The region of Houla, in the west of Syria, comprises several villages and small towns
- The village of Taldou lies around 2km south-west of the main town, also called Houla
- The area is in the province of Homs, which has seen heavy fighting in recent months
- Houla's villages are predominantly Sunni Muslim, but the region is ringed by a number of Alawite villages - the sect of the Syrian president Bashar al-Assad
"These acts may amount to crimes against humanity and other international crimes, and may be indicative of a pattern of widespread or systematic attacks against civilian populations that have been perpetrated with impunity."
Ms Pillay called on the Human Rights Council to "make all efforts to end impunity, to ensure accountability for perpetrators, and to provide adequate and effective remedies for the victims", and urged the Syrian government to protect civilians.
"I reiterate that those who order, assist, or fail to stop attacks on civilians are individually criminally liable for their actions. Other states have a duty to do all they can to prevent and prosecute perpetrators of international crimes."
"Once again, I urge the [UN] Security Council to consider referring the case of Syria to the International Criminal Court."
On Thursday, a Syrian government investigation into the Houla massacre blamed armed rebel groups seeking to trigger foreign military intervention.
The US permanent representative to the UN, Susan Rice, dismissed the finding as a "blatant lie", for which there was no factual evidence.
A resolution before the Human Rights Council condemns "the wanton killings of civilians by shooting at close range and by severe physical abuse by pro-regime elements and a series of government artillery and tank shellings of a residential neighbourhood", and demands that Syria allow in human rights investigators and aid agencies immediately.
But, the BBC's Imogen Foulkes in Geneva says, despite pressure from European countries, there is no call in the draft resolution for Syria to be referred to the ICC.
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Russia and China oppose that, as does the US, which believes only the Security Council should make such a move, our correspondent adds.
Syria is not a state party to the ICC. The court therefore has no jurisdication to indict its citizens without a Security Council referral.
Syria's representative attacked the resolution's sponsors, among them Turkey and Qatar, saying they bore responsibility for some of the deaths, because, he claimed, they had been supplying weapons to rebel groups.
Russian President Vladimir Putin is expected to face pressure over Syria from the leaders of Germany and France when he visits Berlin and Paris. Russia has blocked Security Council action against Syria's government.
UK Foreign Secretary William Hague is meanwhile scheduled to meet representatives of the Syrian opposition in Turkey.
Mr Hague told the BBC that the situation was so grave and deteriorating so rapidly that all options were still on the table.
