Former UN secretary-general Kofi Annan has been appointed as the joint United Nations-Arab League envoy on the Syrian crisis.

Annan's successor, secretary-general Ban Ki-Moon, and Arab League secretary-general Nabil ElAraby announced the appointment.

The announcement said Mr Annan "will provide good offices aimed at bringing an end to all violence and human rights violations, and promoting a peaceful solution to the Syrian crisis".

It said he will engage with parties inside and outside Syria to end the violence and the humanitarian crisis "and facilitate a peaceful Syrian-led and inclusive political solution that meets the democratic aspirations of the Syrian people".

Meanwhile, the United Nations has a secret list of top Syrian officials who could face investigation for crimes against humanity carried out by security forces in their crackdown against an anti-government uprising, a panel of UN human rights experts said.

The UN experts indicated that the list goes as high as president Bashar Assad.

Thousands of Syrians have died in the violence since March and the panel, citing what it called a reliable source, said at least 500 children are among the dead.

"A reliable body of evidence exists that, consistent with other verified circumstances, provides reasonable grounds to believe that particular individuals, including commanding officers and officials at the highest levels of government, bear responsibility for crimes against humanity and other gross human rights violations," said the report by the UN-appointed Independent International Commission of Inquiry on Syria.

It does not say who these investigating authorities might be, but the UN's top human rights official has previously called for Syria to be referred to the International Criminal Court in The Hague.

Members of the 47-nation UN Human Rights Council are expected to hold a special meeting on Syria in Geneva next week, at which the panel's report will be presented. The panel, led by Brazilian professor Paulo Sergio Pinheiro, said its list also identifies some armed opposition cells thought to have committed gross abuses.