UN Secretary General Ban Ki-moon called Monday for all war criminals in Syria to be brought to justice, as his human rights chief urged a probe into the slaughter of hundreds of people in the town of Daraya.
"We must ensure that anyone, on any side, who commits war crimes, crimes against humanity or other violations of international human rights or humanitarian law is brought to justice," Ban said in Geneva, where he opened the 21st session of the United Nations Human Rights Council.
"I am deeply troubled by the aerial bombardments of civilians by government forces, by the increasing sectarian tensions, by the deteriorating humanitarian situation and by the apparent choice of both sides to pursue a solution through force rather than dialogue," he said.
Ban called on the rights council "to maintain its vigilance on Syria, including on the question of accountability".
UN High Commissioner for Human Rights Navi Pillay for her part urged an immediate probe into the slaughter late last month of several hundred people in the Syrian town of Daraya.
"I am deeply shocked by the reports of the massacre in Daraya and I urge an immediate and thorough investigation into this incident," she said on the first day of the three-week rights council session.
"I call on the (Syrian) government to ensure full and unhindered access to the (UN) Independent Commission of Inquiry," she said, also calling for "full support" of the new UN and Arab League peace envoy for the Syria conflict, Lakhdar Brahimi.
Brahimi, who succeeds Kofi Annan in the role, was on Monday meeting with Arab League officials, Egypt's President Mohamed Morsi and other leaders in Cairo to discuss the crisis and finalise plans for a pending visit to Damascus.
Brahimi has described the bloodshed as "staggering" and called the destruction "catastrophic".
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More than 27,000 people have died since the violence began 18 months ago, according to the Syrian Observatory for Human Rights.
In its latest report, published last month, the UN Commission of Inquiry accused the Syrian regime and, to a lesser extent, the opposition forces of committing crimes against humanity.
The commission also announced that it was drawing up a confidential list of people responsible for the atrocities, in a first step towards eventually bringing them to justice.
Before any trials can happen though, numerous difficult steps need to be taken. First, the Human Rights Council will examine the issue in Geneva on September 17.
After that, the UN will need to receive the go-ahead from its members -- for whom consensus on the matter appears all but impossible -- to ask the International Criminal Court to act.
Since the commission headed by Brazilian Paulo Sergio Pinheiro was created a year ago, it has held more than a thousand interviews with perpetrators and victims in the conflict, but has not been able to actually visit Syria.
The commission's mandate will in principle end during the current rights council session, but the diplomats are set in coming days to discuss a new resolution on Syria that will allow the commission's experts to continue their work.
The United States ambassador to the rights council, Eileen Donahoe, said Monday that Washington "strongly supports the extension of the mandate of the commission", lauding it for its "tireless efforts to document these violations and to provide credible and independent reporting on the atrocities in Syria".
The commission's work, she stressed, was "crucial to ensure that the calls for justice from the Syrian people and the international community are not ignored".
She laid the blame for the bloodshed in Syria squarely on the regime, which she said "reacted violently to peaceful demonstrations and... to this day remains bent on denying its people fundamental human rights".
"There can be no doubt that the architect of this destruction is (President) Bashar al-Assad and the regime must end," she said.