What is a war crime? How are suspects tried?

As Aleppo reels from air strikes, UN chief Ban Ki-moon has warned the use of bunker bombs and other advanced munitions against Syria civilians may constitute a war crime.

Here are five facts about war crimes, and the long, arduous legal process to bring perpetrators to justice.

- Definition of a war crime -

Violations of the Geneva Conventions adopted in 1949 following World War II are commonly called "war crimes".

In broad terms, the conventions cover protection of civilians, treatment of prisoners and care for the wounded.

They form the basis of the 1998 Rome Statute, the founding treaty of the world's only permanent court for prosecuting war crimes -- the International Criminal Court (ICC).

Article 8 of the Rome Statute sets out more than 50 examples which could be considered a war crime.

They include wilful killing, torture, taking of hostages, unlawful deportations, intentionally directing attacks against civilians not taking part in hostilities, and deliberately attacking aid and peacekeeping missions.

Using poisonous gases, internationally-banned weapons which cause "superfluous injury or unnecessary suffering or which are inherently indiscriminate" -- such as cluster bombs or incendiary weapons -- or bullets "which expand and flatten easily in the human body" are also considered a war crime.

- Legal history -

International treaties on the laws of war first began being formulated in the mid-1800s. But most such as The Hague Conventions, adopted in 1899 and in 1907, dealt mainly with the treatment of combatants not civilians.

The first high-profile war crimes trials of the modern era were held in Nuremberg and Tokyo in tribunals set up by the Allies to try German and Japanese leaders.

In May 1993, at the height of the Balkans wars, the United Nations established the International Criminal Tribunal for the Former Yugoslavia (ICTY) based in The Hague.

Since its inception, the ICTY has indicted 161 people, of whom 83 have been sentenced, including former Bosnian Serb leader Radovan Karadzic.

Following the genocide in Rwanda, the UN then set up the International Criminal Tribunal for Rwanda in 1994 in Arusha to prosecute those behind the killings of at least 800,000 people.

Both courts highlighted the need for a permanent war crimes tribunal, which gave rise to the ICC.

- Prosecutions at the ICC -

The ICC began work in The Hague in 2003, a year after its statute came into force. To date, 124 countries have signed up to the statute, including 34 from Africa -- the biggest regional group -- and 28 from Latin America and the Caribbean.

A country that has signed up to the treaty or whose citizens have been the victims of crimes may refer cases to the ICC's chief prosecutor, Fatou Bensouda, for investigation.

Cases may also be referred by the United Nations Security Council or the prosecutor can initiate her own investigations with permission from the judges providing member states are involved, or a non-member state can agree to accept the court's jurisdiction.

Any group or individual can report alleged crimes, but it is up to prosecutor to first see whether they fall under her jurisdiction.

So far 23 cases have been brought before the court, and four verdicts -- three guilty, one acquittal -- have been issued.

They include former Congolese militia leader Jean-Pierre Bemba sentenced to 18 years in jail on three counts of war crimes and two charges of crimes against humanity.

Preliminary inquiries or full investigations are also ongoing into situations in 19 countries or territories, with charges yet to be brought.

- The situation with Syria -

Syria is not a signatory to the ICC. Nor are the other major players in the complex conflict -- Russia, the United States, Iran and Saudi Arabia.

As a result, the prosecutor would need a UN mandate to investigate any alleged crimes committed by the government or the rebels in the five-year war in the country -- including the use of chemical weapons.

Attempts to refer Syria to the ICC were vetoed at the UN Security Council in 2014 by Russia and China, to the dismay of human rights groups.

- Will alleged war crimes in Syria ever be tried? -

While the war continues, it is unlikely any prosecutions can be brought before the ICC.

Experts believe accountability will have to be tackled in any eventual peace process. Many argue the best scenario would be some kind of hybrid court based in Syria, but perhaps staffed by a mixture of local and international judges.

SOURCES: International Criminal Court, the Geneva Conventions, the International Committee of the Red Cross (ICRC)