The world witnessed a new horror in Syria this week when more than 2,000 women and children from Islamic State families emerged from a network of tunnels under the town of Baghouz, all starving and many nursing serious injuries.

It has been reported that they were living underground for months, while IS soldiers fought their final battles above ground.

With claims that Kurdish-led forces have now defeated IS in eastern Syria, attention is turning to questions of accountability for the atrocities committed by IS combatants during their deadly fight to build a caliphate in the region.

Human Rights Watch and other reputable sources have documented IS's use of civilians as human shields, employment of landmines and their large scale use of sexually-based crimes, as well as killing and injuring countless civilians.

Who will hold IS to account?

The International Criminal Court (ICC), the world's permanent court established to prosecute the most egregious offences, would seem like the obvious place for current and former ISIS soldiers to be tried for their crimes against Syrians, Kurds and, most devastatingly, the Yazidi population of Northern Iraq, against whom ISIS has been accused of waging genocide.

Islamic State has been accused of waging genocide against the Yazidi sect ( Reuters: Youssef Boudlal )

The ICC certainly has jurisdiction over the crimes in question — war crimes, genocide, crimes against humanity and aggression. It also has an advanced gender justice mandate to bring charges related to rape, sexual slavery, enforced pregnancy and other forms of sexual and gender-based violence known to have been committed by Islamic State rebels against their perceived enemies.

A current case before the ICC may also pave the way to bring charges against militia who commit atrocities against members of their own community — including enslavement or kidnapping of women, or rape.

The ICC can not bring charges

However the ICC does not have jurisdiction over the territory on which these crimes were committed. Neither Iraq nor Syria have signed the ICC's Rome Statute. The ICC Prosecutor, Fatou Bensouda, cannot bring charges herself.

The Rome Statute gives authority to the UN Security Council to refer a matter to the ICC in cases where a state is not party to the Court.

In the Syrian case there is a stumbling block.

The Security Council has voted a dozen times between 2011 and 2018 to bring the Syrian situation before the ICC, and a dozen times Russia, often with China's backing, has vetoed the move.

With the ICC unavailable, what's next for those IS fighters who have broken international criminal laws?

Thousands of women and children from Islamic State families have surrendered after fleeing the group's last stronghold. ( ABC News: Adam Harvey )

Some states may choose to use what is known as "universal jurisdiction" with any IS fighter who resides in, or passes through their territory.

This legal principle allows states to claim criminal jurisdiction over an accused person regardless of where the alleged crime was committed, and regardless of the accused's nationality, country of residence, or any other relation with the prosecuting entity.

Some states in Europe, including Austria, Finland, France, Germany and Sweden, have already invoked this principle in regard to the Syrian conflict, with a number of non-IS perpetrators facing prosecution for terrorism and other crimes.

At the international level, the UN has learnt its lesson about not acting quickly enough to document atrocities in other conflicts. In response the International, Impartial and Independent Mechanism was established in Syria.

Steps to accountability

Set up in Geneva in 2016, the Syrian Mechanism, as it is known, has the task of collecting evidence of crimes by all parties reaching back to 2011.

The Syrian Mechanism will search for evidence of atrocities. ( Reuters: Ari Jalal )

Deputy director of the Syrian Mechanism is Michelle Jarvis, an Australian expert on sexually violent crimes. She is gathering evidence on these violations, which is the first step towards accountability.

The Mechanism will not prosecute IS fighters but will gather evidence for any national, regional or international tribunal established to address the atrocities committed during this conflict.

With evidence at hand, and the end of the conflict waged by IS in sight, it is critical that the international community refocuses its efforts to deliver justice to fighters responsible for a range of heinous crimes.

While the ICC remains out of range, the UN should consider the creation of an ad-hoc tribunal to prosecute crimes committed in the name of IS, similar to those created to address the Rwandan Genocide and the Balkans conflict.

In doing so, the UN must ensure that this time around any tribunal ends impunity for crimes against both men and women that IS inflicted on gender grounds, building on the evidence being gathered by the Syrian Mechanism.

Scientia Professor Louise Chappell is director of the Australian Human Rights Institute at UNSW Sydney. Louise's research focuses on institutional gender justice in local, national and international settings.