A video which appears to show a Syrian rebel taking a bite from the heart of a dead soldier has brought strong condemnation.
US-based Human Rights Watch identified the rebel as Abu Sakkar, a well-known insurgent from the city of Homs, and said his actions were a war crime.
The main Syrian opposition coalition said he would be put on trial.
The video, which cannot be independently authenticated, seems to show him cutting out the heart.
"I swear to God we will eat your hearts and your livers, you soldiers of Bashar the dog," the man says referring to President Bashar al-Assad as he stands over the soldier's corpse.
Human Rights Watch (HRW) says Abu Sakkar is the leader of a group called the Independent Omar al-Farouq Brigade.
"The mutilation of the bodies of enemies is a war crime. But the even more serious issue is the very rapid descent into sectarian rhetoric and violence," HRW's Peter Bouckaert told Reuters news agency.
HRW said those committing war crimes on either side had to know that there was no impunity and that they would be brought to account.
The human rights group said Abu Sakkar had been filmed before, firing rockets into Shia areas of Lebanon and posing with the bodies of guerrillas from the Lebanese Hezbollah movement killed fighting alongside Syrian government forces.
The video, posted on Sunday, is one of the most gruesome to emerge among the many thrown up by more than two years of carnage in Syria, says the BBC's Jim Muir in Beirut.
The UN says 70,000 people have been killed since the uprising against President Bashar al-Assad began in March 2011. The UK-based Syrian Observatory for Human Rights puts the death toll at more than 80,000.
Many Syrians have fled the country to escape the fighting and more than a million are registered as refugees, according the UN. At least 300,000 are estimated to be living in Turkey.
The conflict in Syria will be at the centre of talks in Russia between President Vladimir Putin and Israeli PM Benjamin Netanyahu in the Black Sea resort of Sochi.
Russia is concerned by Israeli air strikes on targets inside Syria, while Israel is unhappy at shipments of Russian weapons to Damascus.
Netanyahu is expected to request that Russia stop supplying the Syrian military with advanced weapons systems. Recent deliveries have included air defence missiles and artillery systems.
Last week, Russian Foreign Minister Sergei Lavrov said Moscow was not violating any international sanctions and would honour already signed contracts, but avoided confirming reports that it was preparing to sell Damascus S-300 air defence batteries.
Earlier this month, the Syrian government accused Israel of bombing military facilities near Damascus. Israel declined to comment, but security sources said the air strikes had been aimed at preventing the transfer of Iranian-made missiles to Hezbollah in Lebanon.
Following the attacks, the Russian foreign ministry warned that the "further whipping-up of armed confrontation" sharply increased the risk of "pockets of tension" in Syria and Lebanon.
In recent days, Russia has been at the centre of diplomatic efforts to end the conflict.
President Putin has had talks on Syria with UK Prime Minister David Cameron and the US Secretary of State, John Kerry. Kerry and Lavrov have also agreed to work towards convening a new international conference to find a political solution.
It will try to convince both the Syrian government and opposition to accept a solution based on the core elements of the final communique issued on 30 June 2012, after the UN-backed Action Group for Syria meeting.
The communique called for an immediate cessation of violence and the establishment of a transitional government that could include officials serving under President Assad and members of the opposition.
Syrian Information Minister Omran al-Zoubi said on Tuesday that Syria would need more information before deciding whether to attend such a conference but would not be party to any meeting which harms "national sovereignty".
He said the future role of Assad was an issue "only for the Syrian people and the ballot box".
On Monday, Prime Minister Cameron and US President Barack Obama expressed hope that Russia would help persuade President Assad to step down.
"As a leader on the world stage, Russia has an interest, as well as an obligation," Obama told reporters in Washington, though he admitted that there remained "lingering suspicions" about Moscow's commitment to ending Mr Assad's rule.
Obama also sounded a note of caution about the new conference, which US officials said was likely to be held in early June and not at the end of May as hoped.
"I'm not promising that it's going to be successful," he said, adding that "once the furies have been unleashed", it is "very hard to put things back together".
Cameron said the UK would call for greater flexibility in the EU arms embargo on Syria, and that it would double non-lethal aid to the rebels, shipping armoured vehicles, body armour and generators.