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As Ban Ki-moon steps down as head of the UN, Syria appears to be weighing heavily on the mind of the South Korean statesman. After 10 years at the helm of the UN he will be replaced by former Portuguese Prime Minister Antonio Guterres at the end of the year.

Addressing the UN Security Council in New York on Wednesday the secretary general spoke with palpable emotion about the ongoing devastation in Syria, describing the inability of the UN to prevent the escalation of the situation in the country - from peaceful protests in 2011 to full-scale civil war shortly after - as his "deepest regret" from his tenure as UN chief.

"My deepest regret on leaving office is the continuing nightmare in Syria,” said Ban, whose address coincided with Syrian President Bashar al-Assad's declaration of "victory" over rebel-forces in Aleppo - a once beautiful city reduced to rubble.

"I once again plead with all of you to cooperate and fulfil your collective responsibility to protect Syrian civilians."

Such a heavy heart is understandable.

Regrets and ongoing concerns

The UN has struggled to stymie the ongoing bloodshed in Syria, often hamstrung by Russian and Chinese vetoes in its Security Council.

It has faced ever-growing accusations of legitimising Assad by coordinating investigations into the use of chemical weapons, and emergency relief procedures targeting besieged communities through Damascus. 

Additional criticism has also been levelled at the organisation for employing local staff with familial and professional affiliations with Syria's state apparatus, and filling the regime's coffers in the process.

Despite fervent criticism of the Syrian regime, and opposition forces, the UN's credibility has principally been compromised by its inability to match criticism with actions that alter realities on the ground.

Hospitals, schools, and water infrastructures have all been targeted by the Syrian regime with ruthless abandon, while opposition groups have also attacked such facilities.

Barrel-bombs, bunker busting bombs, naval mines, and chemical agents such as white phosphorus have all been used in attacks on residential areas.

Meanwhile medieval siege tactics have become a norm in order to starve communities into submission.

In discussions with °®Âþµº over the past few months - as regime forces advanced on East Aleppo - terrified residents often questioned the point of referring to events, grotesque acts of violence claiming innocent lives, as "war crimes".

They said designating parties with such labels did not prevent their re-occurrence or alter any realities on the ground.

Failure to change realities on the ground

Earlier this year the UN estimated that around 400,000 people had been killed in Syria's civil conflict.

But in reality the UN does not really know. It stopped counting the dead in early 2014 when the figure stood at around 250,000.

At that time realities on the ground in Syria were already so fraught and hazardous that verifying daily, rising death tolls had become almost impossible given restricted access.

Yet conflict continues unabated, the death toll ever rising. 

Speaking on growing frustrations with the UN Security Council in September Ban cut a somewhat discontented figure.

"We need some solidarity, unity of purpose, particularly among the permanent members of the Security Council  When they are divided, it is extremely difficult for the United Nations to deliver."

In recent weeks, as barrel bombs pummelled East Aleppo into oblivion, before a ceasefire and stalling evacuation (negotiated by Russia and Turkey, away from the UN's oversight) came into place the UN appeared impotent to effect change on the ground, unable to establish culpability for grave acts of violence targeting civilian populations. 

Instead the organisation, established as an intergovernmental body aimed at promoting international cooperation in the aftermath of World War II in order to prevent another such conflict, was reduced simply to paying lip service to accountability and war crimes.

One of the UN's central functions is to highlight human rights abuses and contraventions of international laws governing warfare, and ensure that culpability mechanisms are in place to prevent their continued practice.

In Ki-moon's own words, in Syria, the UN has struggled to "deliver". 

Like the League of Nations before it, it is at risk of looking increasingly defunct. 

As the secretary general retires with a heavy heart his successor is more than aware of the difficulties that await him. Antonio Guterres served as head of UNHCR - the UN refugee agency - between 2005 and 2015.

With the UN's global credibility looking increasingly shaky, and a war that has baulked and scorned  at talk of war crimes and contraventions of international law continuing to rage on many fronts, inheriting the Syria file appears arguably an unenviable position to be in.