(CNN) The bombardment of rebel-held districts of eastern Aleppo this week -- hours after a truce had been declared -- prompted grave claims that a war crime had been committed.

Turkey's President Recep Tayyip Erdogan accused the government of Syrian President Bashar al-Assad of "brazenly committing war crimes." UN human rights chief Zeid Ra'ad al Hussein similarly said the Syrian regime's heavy bombardment of an area "packed with civilians" after the brief ceasefire broke down "most likely constitutes war crimes."

In a tweet late Thursday, Ukrainian President Petro Poroshenko wrote that "Russia must also pay a high price for heinous war crimes, which it supports and commits in Syria, in particular in Aleppo."

It's not the first time such accusations of war crimes have been made in Syria's protracted conflict. Opposition fighters were last week accused of blocking civilians from fleeing rebel-held areas as pro-regime forces advanced, and reportedly abducting and killing some civilians who asked them to quit their neighborhoods.

But what is the definition of a war crime -- and will anyone be held to account?

What constitutes a war crime?

In the midst of war, international rules protect those who are not active participants in the fighting, such as civilians and prisoners of war.

They include international human rights law that applies at all times, whether a country is at war or at peace, and international humanitarian law , such as set out in the Geneva Conventions, which applies in situations of armed conflict.

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An older body of rules, the Laws and Customs of War, also governs the actions of warring parties, as do domestic laws and international human rights treaties that states have signed up to, such as the UN Convention against Torture.

Mark Ellis, executive director of the International Bar Association, which brings together lawyers worldwide, told CNN that the Geneva Conventions make clear that civilians have to be protected by all combatants.

These include: wilful killing; torture or inhuman treatment; wilfully causing great suffering, causing serious injury to body or health; extensive destruction and appropriation of property, not justified by military necessity and carried out unlawfully and wantonly; hostage taking; and wilfully making the civilian population or individual civilians the object of attack, if death or serious injury is caused.

Have war crimes been committed in Syria?

No war crimes prosecution has yet been brought against individuals in Syria -- but there have been many allegations of wrongdoing against both sides.

"I don't think there's any doubt that war crimes have been committed on a very, very serious scale in Aleppo and elsewhere in Syria," said Avner Gidron, senior policy and law adviser for rights group Amnesty International. "Crimes against humanity are also being committed in Syria, not only war crimes."

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The war crimes don't just include bombing attacks that kill civilians, Gidron said, but also abuses being committed in detention centers and prisons. "

Civilians cannot be made the target of an attack under the Geneva Conventions, Ellis said, but in Syria "the evidence suggests that it's just the opposite."

Who can be accused of war crimes?

Central to the idea of war crimes is that an individual can be held criminally responsible for carrying out an unlawful act or, significantly, for ordering it, Gidron said.

Those assessing whether civilians are the victim of a war crime would need to establish details such as what the target of an attack was, what information the commander had about any risk to civilians, what precautions were taken to prevent harm to civilians, and whether the risk of harm to civilians was disproportionate even if it was a legitimate military target, Gidron said.

"These are very hard things to assess," he said, adding that international law allows some leeway for the realities of war.

Who can bring a war crimes case to justice?

The International Criminal Court was set up to try individuals for genocide, war crimes and crimes against humanity.

However, a case would normally have to be referred to it by the UN Security Council.

"This is quite unlikely in this case because of Russia's position on the Security Council," said Ellis. Russia, as a permanent Security Council member, has veto power which it has already used to block UN resolutions on Syria.

Another possibility involves the principle of "universal jurisdiction."

Syrian pro-government forces advance during a military operation in Aleppo on Wednesday.

Some offenses, such as war crimes, crimes against humanity and genocide, are so serious that individual states can seek to prosecute them in their national courts even if they are not party to the conflict and their own citizens are not involved.

What's not yet clear is whether any state would have the political will to bring such a case.

In the past, Spain has been most active worldwide in pursuing cases under the principle of universal jurisdiction.

There has also been discussion of a possible post-war tribunal to try alleged war crimes in Syria, Ellis said, "but I think that's unlikely if Assad stays in power."

Is evidence of alleged war crimes being collected in Syria?

Independent observers may have very limited access to Syria, but those committing war crimes need not rest easy in their beds.

International rights groups such as Amnesty International and Human Rights Watch, as well as local Syrian activist groups, are working to preserve evidence and gather testimonies, said Gidron.

Buses used to evacuate people from rebel-held areas of Aleppo are seen waiting on Thursday.

A spokesman for the UK Foreign Office said it had "trained and equipped the Syrian moderate opposition to collect evidence in Syria of human rights violations and abuses, as well as evidence of breaches of violations of international humanitarian law by any party."

And UK Foreign Office Minister for the Middle East, Tobias Ellwood, tweeted Wednesday that Russia's use of its veto power at the UN Security Council "will not prevent individuals committing potential crimes against humanity from being held to account."

#Aleppo : As Russia abuses its veto at the #UNSC - we must work to hold to account those participating in potential war crimes. pic.twitter.com/oslCS9H8IP — Tobias Ellwood MP (@Tobias_Ellwood) December 14, 2016

The wealth of video and pictorial evidence that can be collected today would never have been available before, said Ellis, and represents "a very powerful part of building a case."

Will anyone be prosecuted for war crimes in Syria?

As and when the conflict comes to an end, international attention may turn from the current efforts to alleviate civilian suffering in Syria to bringing those responsible to justice, said Ellis.

But Gidron warned that it can take a long time for those accused of war crimes to be held accountable.

"We are just seeing in Latin America now people being brought to justice for atrocities 40 years ago, still seeing Nazi war crime suspects being tried after all these years," said Gidron.

"It takes a long time sometimes but it's important to try to preserve the evidence of the atrocities, so there's a chance of this later on."

Jan Egeland, special adviser to the UN Special Envoy for Syria, pointed the finger Thursday at all parties for hindering action by the international community.

"They have not been able to give us the access that we needed and therefore we have not been witnesses to atrocities that we know have been committed by all sides in this horrific war, including in east Aleppo."